Method for making group III nitride articles

ABSTRACT

Group III (Al, Ga, In)N single crystals, articles and films useful for producing optoelectronic devices (such as light emitting diodes (LEDs), laser diodes (LDs) and photodetectors) and electronic devices (such as high electron mobility transistors (HEMTs)) composed of III-V nitride compounds, and methods for fabricating such crystals, articles and films.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 60/749,728, filed Dec. 12, 2005, titled “BulkGallium Nitride Crystals and Method of Making the Same;” U.S.Provisional Patent Application Ser. No. 60/750,982, filed Dec. 16, 2005,titled “Method of Producing Freestanding Gallium Nitride bySelf-Separation;” U.S. Provisional Patent Application Ser. No.60/810,537, filed Jun. 2, 2006, titled “Low Defect GaN Films Useful forElectronic and Optoelectronic Devices and Method of Making the Same;”U.S. Provisional Patent Application Ser. No. 60/843,036, filed Sep. 8,2006, titled “Methods for Making Inclusion-Free Uniform Semi-InsulatingGallium Nitride Substrate;” and U.S. Provisional Patent Application Ser.No. 60/847,855, filed Sep. 28, 2006, titled “Method of Producing SingleCrystal Gallium Nitride Substrates by HVPE Method Incorporating aPolycrystalline Layer for Yield Enhancement,” the contents of each ofwhich are incorporated by reference herein in their entireties. Thecontent of International Patent Application No. PCT/US2006/045965 filedNov. 30, 2006, titled “Group III Nitride Articles and Methods for MakingSame” is incorporated by reference in its entirety.

FEDERALLY SPONSORED RESEARCH

This invention was made with government support under grant numberN00164-04-C-6066 by the Missile Defense Agency (“MDA”). The UnitedStates Government may have certain rights in the invention.

BACKGROUND

1. Field of the Invention

This invention relates to Group III (Al, Ga, In)N articles (e.g.,crystals, boules, substrates, wafers, layers, films, and the like)useful for producing optoelectronic devices (such as light emittingdiodes (LEDs), laser diodes (LDs) and photodetectors) and electronicdevices (such as high electron mobility transistors (HEMTs)) composed ofIII-V nitride compounds, and to methods for producing such articles.

2. Description of the Related Art

Group III-V nitride compounds such as aluminum nitride (AlN), galliumnitride (GaN), indium nitride (InN), and alloys such as AlGaN, InGaN,and AlGaInN, are direct bandgap semiconductors with bandgap energyranging from about 0.6 eV for InN to about 6.2 eV for AlN. Thesematerials may be used to produce light emitting devices such as LEDs andLDs in short wavelength in the green, blue and ultraviolet (UV) spectra.Blue and violet laser diodes may be used for reading data from andwriting data to high-density optical storage discs, such as those usedby Blu-Ray and HD-DVD systems. By using proper color conversion withphosphors, blue and UV light emitting diodes can be made to emit whitelight, which may be used for energy efficient solid-state light sources.Alloys with higher bandgaps may be used for UV photodetectors that areinsensitive to solar radiation. The material properties of the III-Vnitride compounds are also suitable for fabrication of electronicdevices that may be operated at higher temperature, higher power, andhigher frequency than conventional devices based on silicon (Si) orgallium arsenide (GaAs).

Most of the III-V nitride devices are grown on foreign substrates suchas sapphire (Al₂O₃) and silicon carbide (SiC) because of the lack ofavailable low-cost, high-quality, large-area native substrates such asGaN substrates. Blue LEDs are mostly grown on insulating sapphiresubstrates or conductive silicon carbide substrates using ametal-organic chemical vapor deposition (MOCVD) process. Sapphirebelongs to the trigonal symmetry group, while SiC belongs to thehexagonal symmetry group. GaN films and InGaN films have beenheteroepitaxially grown on the c-plane sapphire surface for LED devices.Due to lattice mismatch, the GaN films grown on both sapphire and SiCsubstrates typically have high crystal defects with a dislocationdensity of 10⁹ to 10¹⁰ cm⁻³. Despite the high defect density of the LEDsgrown on these substrates, commercial LEDs have long lifetimes suitablefor some applications.

The MOCVD process is a slow growth rate process with a growth rate of afew microns per hour. In a typical GaN-based device growth process, alow-temperature buffer layer of GaN or Al_(x)Ga_(1-x)N (x=0-1) is firstgrown on a foreign substrate (e.g., sapphire or silicon carbide),followed by the growth of a few microns of GaN. The active device layer,such as quantum well structures for LEDs, is subsequently grown. Forexample, U.S. Pat. No. 5,563,422 to S. Nakamura et al. describes aGaN-based device grown by an MOCVD process. A thin GaN nucleation layerof about 10 nanometers is first deposited on a sapphire substrate at alow temperature of 500-600° C. The GaN nucleation layer is annealed athigh temperature to recrystallize the GaN, and epitaxial GaN film isgrown at higher temperature (approximately 1000-1200° C.).

UV LEDs based on alloys of GaN, however, show strong dependence of thepower output on the substrate material used. The UV LEDs can be grown onnative GaN substrates or on foreign substrates such as sapphire andsilicon carbide. On the foreign substrate, a GaN or AlGaN thin film isfirst grown by utilizing appropriate techniques and the active UV LEDstructure is subsequently grown. It has been found that the power outputof UV LEDs grown on native GaN substrates is much greater than the poweroutput of those grown foreign substrates (see, for example, Yasan et al.Applied Physics Letters, Volume 81, pages 2151-2154 (2002); Akita et al.Japanese Journal of Applied Physics, Volume 43, pages 8030-8031 (2004)).The lower density of crystal defects in the device structure grown on anative GaN substrate contributes to higher power output.

Group III-V nitride-based laser diodes also show a remarkable dependenceof lifetime on the crystal defect density. The lifetime of the LDsdramatically decreases with the increase of the dislocation density(see, for example, “Structural defects related issues of GaN-based laserdiodes,” S. Tomiya et al., MRS Symposium Proceedings, Vol. 831, p. 3-13,2005). Low defect density single-crystal gallium nitride substrates areneeded for the long lifetime (>10,000 hours) nitride laser diodes. ForLEDs based on an AlGaN active layer operating at the deeper UV range, itis also found that dislocation density has a detrimental effect on theperformance and lifetime of the devices. For LEDs operating at higherpower levels, it is also desirable to have a lower defect density GaNlayer.

Because of the very high equilibrium nitrogen pressure at the meltingpoint, gallium nitride single crystals cannot be grown with conventionalcrystal growth methods such as the Bridgman method or Czochralski methodwhere single crystals are grown from the stoichiometric melt. At ambientpressure, GaN starts to decompose well before melting.

Despite such difficulties, small-size high-quality single-crystal GaNsubstrates have been made with several methods. Porowski et al.discloses a method of growing bulk GaN at high nitrogen pressure (S.Porowski and I. Grzegory, J. Cryst. Growth, Vol 178, 174 (1997)).Gallium metal is reacted with gaseous nitrogen at a nitrogen pressure ashigh as 20 kbar and a temperature as high as 2000 K. The crystal growthrate of the process is slow and long growth times of 60-150 hours areneeded to produce crystal platelets of about 1 cm in length and about0.1 mm in thickness. It is also believed that the process is notscalable, i.e., longer growth times or larger reactors cannot increasethe crystal size substantially. Disalvo et al. in U.S. Pat. No.5,868,837 discloses a method of GaN growth using sodium flux wheregallium metal reacts with gaseous nitrogen under moderate temperatureand pressure. However, the GaN crystals grown with the sodium fluxmethod are small, about a few millimeters in size. D'Evelyn et al. inU.S. Pat. App. Pub. Nos. 2004/0124434 and 2005/0098095 discloses thegrowth of GaN in supercritical ammonia (NH₃). Although high crystalquality can be achieved, the growth rate may be quite slow, and thusproduces crystals with small area. Therefore, GaN crystal growth fromliquid phase, in general, yields small crystals, and is not suitable forcommercial applications.

Large-area GaN crystals may be grown by hydride vapor phase epitaxy(HVPE) methods. In the HVPE process, gallium chloride (GaCl), formed byreacting gaseous hydrochloric acid (HCl) with gallium metal in theupstream of the reactor, reacts with ammonia (NH₃), depositing GaN onthe surface of a substrate. The size of the GaN crystal grown may be thesame as the size of the substrate. Substrates such as sapphire, galliumarsenide, silicon carbide, and other suitable foreign substrates havebeen used. Since large-sized substrates with a diameter from 2 inches to12 inches are available, large-sized GaN, in theory, could be grown withHVPE techniques. However, bulk GaN growth on sapphire substrate by HVPEencounters many obstacles, such as nucleation of polycrystallinematerial, cracking and microcracking during growth and cool down, and anunstable crystal growth front that leads to polycrystalline formation ormicrocracking during the bulk growth.

Bulk GaN growth methods, in which multiple wafers can be produced, cansignificantly reduce the cost the GaN wafer manufacturing. The materialquality also improves with the bulk growth. Vaudo et al. in U.S. Pat.No. 6,596,079 discloses a vapor phase method for growing a GaN bouleusing native GaN crystal as a seed. However, to practice the invention,a high-quality GaN seed is first required. Melnik et al. in U.S. Pat.No. 6,616,757 discloses a similar method for growing GaN crystal bouleby hydride vapor phase epitaxy. A GaN single-crystal layer is firstgrown on a silicon carbide substrate, the substrate is subsequentlyremoved by etching in molten KOH to form a GaN seed, and a GaN boule isgrown on the seed to a length greater than 1 centimeter. However,silicon carbide substrates are of higher cost than sapphire substratesand the process is not applicable to sapphire substrate since sapphirecannot be etched away. Motoki et al. in U.S. Pat. Nos. 6,413,627;6,468,347; and 6,667,184 discloses methods for growing a bulk GaNcrystalline boule by HVPE on GaAs and on native GaN seeds. Because ofthe processes used, the GaN crystal boule grown by Motoki's methodscontain clusters of highly defective material.

In view of such prior-art approaches to bulk GaN crystal growth, it iswell-acknowledged that there is still a need in the art for low-costmethods for manufacturing high-quality GaN crystal boules and wafers.

It is also of interest to produce free-standing GaN articles byseparating or removing the underlying substrate. Vaudo et al. in U.S.Pat. No. 6,440,823 discloses a method for producing low defect GaN usingHVPE on sapphire substrates. The sapphire substrate can be removed toproduce large-area GaN substrate, for example, by a laser inducedliftoff process as described by Kelly et al. (“Large freestanding GaNsubstrates by hydride vapor phase epitaxy and laser-induced liftoff,”Jpn J. Appl. Phys., Vol. 38, L217-L219, 1999). The wavelength of thelaser beam, or the energy of the laser beam, is chosen so that it issmaller than the bandgap of the substrate, but larger than the bandgapof GaN. The substrate is transparent to the laser beam, but the GaNabsorbs the laser energy, heating the interface and decomposing the GaNat the interface, which separates the GaN film from the substrate. InU.S. Pat. App. Pub. No. 2002/0068201, Vaudo et al. further discloses amethod for producing freestanding GaN near the growth temperature byshining a laser beam at the interface between the grown GaN layer andthe template, and decomposing the interface material. This processinvolves dangerous high-energy laser beams and high manufacturing cost.Park et al. in U.S. Pat. No. 6,652,648 discloses a similar method forproducing GaN substrate by first growing HVPE GaN on sapphire substratesand followed by laser liftoff. Motoki et al. in U.S. Pat. No. 6,693,021discloses a method for growing a thick GaN film on a gallium arsenide(GaAs) substrate, in which the GaAs substrate was wet-etched away toproduce a free-standing GaN substrate.

The known methods for producing GaN substrates such as disclosed in U.S.Pat. Nos. 6,693,021; 6,652,648; and 6,440,823 can only yield one waferper growth run, and thus are of high manufacturing cost.

Chin Kyo Kim in U.S. Pat. No. 6,923,859 discloses an apparatus andassociated manufacturing method for GaN substrates in which a substrateand a GaN layer are separated from each other after growing the GaNlayer on the substrate in the same chamber. The apparatus contains atransparent window at the circumference of the chamber to allow theintroduction of the laser beam to the substrate. This process likewiseinvolves dangerous high-energy laser beams and has high manufacturingcost.

Bong-Cheol Kim in U.S. Pat. 6,750,121 discloses an apparatus and methodfor forming a single crystalline nitride substrate using hydride vaporphase epitaxy and a laser beam. After growth of the GaN film on sapphiresubstrate, the wafer is moved to a heated chamber for laser-introducedseparation. Because the wafer does not cool to room temperature,cracking induced by the mismatch of the coefficient of thermal expansionis eliminated. This process likewise involves dangerous high-energylaser beams and has high manufacturing cost.

Park et al. in U.S. Pat. No. 6,652,648 discloses a method for producingGaN substrate by first growing HVPE GaN on sapphire substrates. Thebackside of the sapphire substrate is protected for minimal parasiticdeposition. After GaN growth on sapphire substrate, the GaN/sapphirestructure is removed from the reactor. The GaN layer is subsequentlyseparated from the sapphire substrate by a laser liftoff process. Inaddition to involving dangerous high-energy laser beams, the GaN layeron sapphire is likely to crack upon cool down, and thus this processsuffers with low yield and high manufacturing cost.

Motoki et al. in U.S. Pat. No. 6,693,021 discloses a method of growingthick GaN film on gallium arsenide (GaAs) substrate. The GaAs substrateis wet-etched away to produce a free-standing GaN substrate. However,GaAs substrates tend to thermally decompose at the GaN growthtemperature and in the GaN crystal growth environment, introducingimpurities to the GaN film.

Yuri et al. in U.S. Pat. No. 6,274,518 discloses a method for producinga GaN substrate. A first semiconductor film (AlGaN) layer is formed on asapphire substrate, and a plurality of grooves is formed on the AlGaNlayer. A relatively thick GaN film is grown on a grooved AlGaN templateby an HVPE method, and upon cooling down from the growth temperature toroom temperature, GaN separates from the template, forming a large areafreestanding GaN substrate. However, this method requires deposition andpatterning of several films in different systems andcracking-separation. Thus, the process is one of low yield and highmanufacturing cost.

Solomon in U.S. Pat. No. 6,146,457 discloses a thermal mismatchcompensation method to produce a GaN substrate. The GaN film isdeposited at a growth temperature on a thermally mismatched foreignsubstrate to a thickness on the order of the substrate, where thesubstrate is either coated with a thin interlayer or patterned. Aftercool down from the growth temperature to the room temperature, it isstated that thermal mismatch generates defects in the substrate, not inthe GaN film, producing a thick high-quality GaN material. However, theGaN material of Solomon's invention is still attached to the underlyingsubstrate, with the underlying substrate containing substantial defectsand/or cracks. Subsequently, other processing steps are required tocreate a freestanding GaN layer.

Usui et al. in U.S. Pat. No. 6,924,159 discloses a void-assisted methodfor manufacturing GaN substrate. In this method, a first GaN thin filmis deposited on a foreign substrate, and a thin metal film such astitanium film is then deposited on the first GaN thin film. The titaniummetal film is heated in hydrogen-containing gas to form voids in thefirst GaN thin film. A thick GaN film is subsequently deposited on thefirst void-containing GaN film. The voids in the first GaN film createfracture weakness, and upon cooling from the growth temperature toambient room temperature, the thick GaN layer separates itself from thesubstrate, forming a GaN wafer. However, this method requires depositionof several layers of films in different systems and cracking-separation.Thus, the process is one of low yield and high manufacturing cost.

The techniques of the prior art for manufacturing GaN wafers areattended by high manufacturing cost. There are some commercial vendorscurrently selling 2″ GaN wafers, but at very high price, reflecting thehigh manufacturing cost. Additionally, researchers have shown that thefreestanding GaN substrates formed using the laser-induced liftoffprocess can be subject to substantial bowing, which limits theirusability for device manufacturing (see, for example, “Growth of thickGaN layers with hydride vapour phase epitaxy,” B. Monemar et al., J.Crystal Growth, 281 (2005) 17-31).

In view of such prior-art approaches to forming GaN substrates, it iswell-acknowledged that there is still a need in the art for low-costmethods for producing high-quality free-standing GaN substrates.

There is also interest in fabricating electronic devices in which anactive layer is built on GaN film. Because of the lattice mismatchbetween gallium nitride and the non-native substrate, there is a largenumber of crystal defects in the GaN film and active device layer. Thedefect density in the GaN nucleation layer is thought to be on the orderof 10¹¹ cm⁻² or greater, and in the subsequently grown GaN layer andactive device layer, the typical density of crystal defects, inparticular, the threading dislocation density, is on the order of10⁹-10¹⁰ cm⁻² or greater in typical GaN-based LEDs as previously noted.Moreover, because of the large mismatch in both the thermal expansioncoefficients and the lattice constants of the foreign substrate and theGaN film, problems such as a high defect density lead to short devicelifetime and bowing of GaN/heteroepitaxial substrate structures. Bowingleads to difficulty in fabricating devices with small feature sizes.

For LEDs based on an AlGaN active layer operating at the deeper UVrange, it is also found that dislocation density has a detrimentaleffect on the performance and lifetime of the devices. For LEDsoperating at higher power levels, it is also desirable to have a lowerdefect density GaN layer.

There are several growth methods that may possibly be performed toreduce the defect density of the gallium nitride film. One commonapproach in MOCVD growth of gallium nitride is epitaxial lateralovergrowth (ELOG) and its variations. In an ELOG GaN growth process, aGaN film is first grown by a MOCVD method with the 2-step process(low-temperature buffer and high-temperature growth). A dielectric layersuch as silicon oxide or silicon nitride is deposited on the first GaNfilm. The dielectric layer is patterned with a photolithographic methodand etched so that portions of GaN surface are exposed and portions ofthe GaN film are still covered with the dielectric mask layer. Thepatterned GaN film is reloaded into the MOCVD reactor and growth isre-commenced. The growth condition is chosen such that the second GaNlayer can only be grown on the exposed GaN surface, but not directly onthe masked area. When the thickness of the second GaN layer is thickerthan the dielectric layer, GaN can grow not only along the original cdirection, but also along the sidewalls of the GaN growing out of theexposed area and gradually covering the dielectric mask. At the end ofthe growth, the dielectric mask will be completely covered by the GaNfilm and the GaN film overall is quite smooth. However, the distributionof the threading dislocation density is not uniform. Since thedislocation density of the first GaN layer is quite high, the defectdensity is also high in the area of the second GaN layer grown directlyon the exposed first GaN layer. In comparison, the defect density ismuch reduced in the area above the dielectric mask area where the secondGaN layer was grown laterally in the direction parallel to the surface.The defect density is still high in the area where the second GaN layerwas grown directly on the first GaN layer and in the area where thelateral grown GaN coalesced. Multiple ELOG processes can be used tofurther reduce the defect density by patterning a second dielectric maskcovering the high defect density areas of the first ELOG GaN film, andgrowing GaN film in the ELOG condition that yields a coalesced secondELOG film.

The manufacturing cost of the prior-art low defect density GaN filmbased on MOCVD is high due to multiple growth and photolithographicsteps. The high cost of the film also increases the overallmanufacturing cost of end products such as UV LEDs.

Therefore, there is still a compelling need in the art for low-costmethods for producing high-quality, low defect density GaN films thatare suitable for electronic and optoelectronic devices to be built on.

Conductive GaN substrates have recently become available. Suchconductive GaN substrates are advantageously employed in themanufacturing of blue and UV lasers. However, in a number of electronicapplications such as high electron mobility transistors (HEMTs), aninsulating or semi-insulating GaN substrate is highly desirable.

Unintentionally doped GaN exhibits n-type conductivity due to thepresence of residual n-type impurities as well as crystal defects. SinceGaN has a high bandgap energy, a pure and defect-free GaN materialshould exhibit insulating or semi-insulating electric properties.However, current GaN crystal growth techniques still allow theunintentional incorporation of impurities and various crystal defectssuch as vacancies and dislocations, which render the GaN crystalsconductive.

It is known in the prior art that by introducing deep-level compensatingimpurities in the crystal, a wide bandgap semiconductor can be madesemi-insulating. For example, U.S. Pat. No. 5,611,955 issued to Barrettet al. discloses the use of vanadium doping in silicon carbide toproduce a semi-insulating SiC crystal. Similarly, Beccard et al.discloses the use of iron chloride formed by reacting elemental ironwith gaseous hydrochloric acid in a vapor phase reactor during the HVPEgrowth of indium phosphide (InP) to produce iron-doped semi-insulatingInP crystals (R. Beccard et al., J. Cryst. Growth, Vol. 121, page373-380, 1992). The compensating impurities act as deep-level acceptorsto trap the otherwise free electrons generated by unintentionally dopedn-type impurities and crystal defects. The concentration of thedeep-level acceptor is typically higher than the concentration of thefree electrons generated by the n-type impurities and crystal defects.

Several deep-level acceptors generated by compensating impurities ingallium nitride (GaN) have been identified in the prior art. Forexample, Group II metals such as Be, Mg, and Zn, and transition metalssuch as Fe and Mn, can be incorporated in the GaN crystal resulting insemi-insulating electric properties. The energy level of iron in galliumnitride is well-documented and iron incorporation can result in galliumnitride exhibiting the semi-insulating electric property (see, forexample, R. Heitz et al., Physical Review B, Vol. 55, page 4382, 1977).Iron-doped gallium nitride thin films can be grown with metal-organicchemical vapor deposition, molecular beam epitaxy, and hydride vaporphase epitaxy (see, for example, J. Baur et al., Applied PhysicsLetters, Vol. 64, page 857, 1994; S. Heikman, Applied Physics Letters,Vol. 81, page 439, 2002; and A. Corrion, et al., Journal of CrystalGrowth, Vol. 289, page 587, 2006). Zinc-doped gallium nitride thin filmsgrown by hydride vapor phase epitaxy can be semi-insulating as well (N.I. Kuznetsov et al., Applied Physics Letters, Vol. 75, page 3138, 1999).

U.S. Pat. No. 6,273,948 issued to Porowski et al. discloses a method formaking a highly resistive GaN bulk crystal. The GaN crystal is grownfrom molten gallium under an atmosphere of high-pressure nitrogen(0.5-2.0 GPa) and at high temperature (1300-1700° C.). When pure galliumis used, the GaN crystal grown is conductive due to residual n-typeimpurities and crystal defects. When a mixture of gallium and a Group IImetal such as beryllium, magnesium, calcium, zinc, or cadmium is used,the grown GaN crystal is highly resistive, with a resistivity of 10⁴-10⁸ohm-cm. However, the crystals obtained from molten gallium under thehigh-pressure, high-temperature process were quite small, on the orderof one centimeter, which is not suitable for most commercial electronicapplications.

U.S. Pat. App. Pub. No. 2005/0009310 by Vaudo et al. discloses alarge-area semi-insulating GaN substrate grown by hydride vapor phaseepitaxy (HVPE). Typically, undoped HVPE-grown GaN is of n-typeconductivity due to the residual impurities and crystal defects. Byintroducing a deep-level doping species during the growth process and ata sufficiently high concentration of the dopant species in the GaNcrystal, the grown GaN crystal becomes semi-insulating. Typical dopantspecies are transitional metals such as iron.

However, during the HVPE growth of single-crystal GaN, there are varioussurface morphologies observed and these different growth morphologieshave different levels of impurity incorporation. U.S. Pat. No. 6,468,347by Motoki et al. discloses that in the growth of GaN on c-planesubstrate by HVPE, the growth surface has inverse pyramidal pits.Because of the presence of the pits on the growing GaN surface, theactual GaN growth takes place both on the non-pitted area, which isnormal c-plane growth, and on the faces of the pits, which isnon-c-plane growth. U.S. Pat. Nos. 6,773,504 and No. 7,012,318 by Motokiet al. disclose that GaN growth on the surfaces other than the c-planehas much higher oxygen incorporation.

The presence of inverse pyramidal pits on the GaN crystal surface duringHVPE growth results in a non-uniform distribution of n-type impurityconcentration in the GaN crystal due to higher impurity incorporation onthe non-c-plane surfaces. The impurity concentrations in these pittedareas can be an order of magnitude or more higher than in non-pittedareas. Even when compensating deep-level impurities such as iron areintroduced during the crystal growth, the electric characteristics ofthe grown GaN crystal are not uniform when pits are present during thegrowth, and GaN wafers made from such crystals will have a non-uniformsheet resistance across the wafer surface. When the as-grown GaN ispolished to remove the pits and to produce a smooth surface, theimpurity concentration on the surface is still not uniform. The areaswhere pits were present have a higher oxygen impurity concentration,appear to be darker in color than the surrounding area, and areconsidered as “inclusions” of more conductive spots. Electronic devicesgrown on substrates with non-uniform electric properties have lowerperformance, resulting in lower device yield. Substrates that are“inclusion-free,” or those substrates without non-uniform areas of moreconductive spots, would have a more uniform sheet resistance across thewafer surface and higher performance, resulting in higher device yield.

Therefore, there is also a compelling need in the art for large-area,inclusion-free, uniform semi-insulating GaN substrates and methods formaking such substrates.

SUMMARY

The present invention in some aspects generally relates to galliumnitride (Al, Ga, In)N articles (e.g., crystals, boules, substrates,wafers, layers, films, and the like) and to methods for producing sucharticles.

According to one implementation, a bulk crystal structure includes asubstrate, an AlN epitaxial layer grown on the substrate, a GaNnucleation layer grown on the AlN epitaxial layer, a transitional GaNlayer on the nucleation layer, and a bulk GaN layer grown on thetransitional layer.

According to another implementation, a bulk crystal structure includes asubstrate, a GaN nucleation layer grown on the substrate, a transitionalGaN layer on the nucleation layer, and a bulk GaN layer grown on thetransitional layer.

According to another implementation, a method is provided for growing abulk crystal structure. A GaN nucleation layer is grown on a substrateby HVPE. A transitional GaN layer is grown on the nucleation layer byHVPE. A bulk GaN layer is grown on the transitional GaN layer by HVPE.

According to another implementation, a method is provided for growing abulk crystal structure, in which an AlN epitaxial layer is deposited onthe substrate and the GaN nucleation layer is grown on the AlN epitaxiallayer.

According to another implementation, a method is provided for growing abulk crystal structure. A GaN nucleation layer is grown on a substrateby HVPE under nucleation layer growth conditions including a firstgrowth temperature, a first ammonia partial pressure, a first V:IIIratio, and a first growth rate. A transitional GaN layer is grown on thenucleation layer by HVPE under transitional layer growth conditionsincluding a second growth temperature, a second ammonia partialpressure, a second V:III ratio, and a second growth rate, wherein atleast one of the transitional layer growth conditions is changed fromthe corresponding nucleation layer growth condition. A bulk GaN layer isgrown on the transitional GaN layer by HVPE.

According to another implementation, a method is provided for growing abulk crystal structure, in which the transitional layer growthconditions are selected from the group consisting of the first growthtemperature being increased to the second growth temperature, the firstammonia partial pressure being reduced to the second ammonia partialpressure, the first growth rate being reduced to the second growth rate,and two or more of the foregoing.

According to another implementation, a method is provided for growing abulk crystal structure, in which the nucleation layer growth conditionsproduce a pitted nucleation layer surface, and the transitional layergrowth conditions produce a transitional layer surface having a lesserpercentage of pits than the pitted nucleation layer surface.

The present invention in other aspects generally relates tofree-standing gallium nitride (Al, Ga, In)N articles and methods formaking such articles.

According to one implementation, a method is provided for making afree-standing GaN substrate. An epitaxial nitride layer is deposited ona single-crystal substrate to form a nitride-coated substrate. A 3Dnucleation GaN layer is grown on the epitaxial nitride layer by HVPEunder a substantially 3D growth mode. A GaN recovery layer is grown onthe 3D nucleation layer by HVPE under a condition that gradually changesthe growth mode from the substantially 3D growth mode to a substantially2D growth mode. A bulk GaN layer is grown on the recovery layer by HVPEunder the substantially 2D growth mode to form a GaN/substrate bi-layer.The GaN/substrate bi-layer is cooled from a growth temperature at whichthe bulk layer is grown to an ambient temperature, wherein GaN materialof the bi-layer separates from the substrate to form a substantiallycrack-free free-standing GaN substrate.

According to another implementation, a method is provided forreproducibly growing a GaN single crystal by HVPE. A sapphire substrateis provided. An epitaxial AlN layer is deposited on the substrate. A GaNlayer is grown on the AlN layer by HVPE, wherein the growth temperatureis in the range between 900° C. and 1100° C., the growth rate in therange from about 50 microns per hour to about 500 micron per hours, andthe V:III ratio is in the range between 10 and 100.

According to another implementation, a method is provided for preventingGaN microcracking during the growth of GaN single crystal film on aforeign substrate by HVPE. A substrate is processed wherein thesubstrate is ready for epitaxial GaN growth by HVPE. A first layer ofepitaxial GaN is deposited by HVPE, wherein the first layer is grown ina 3D growth mode. A second layer of GaN is deposited on the first layerby HVPE under a growth mode selected from the group consisting of a 2Dgrowth mode and a 3D growth mode.

According to another implementation, a free-standing GaN substrate isprovided, which is produced according to any one of the foregoingmethods.

The present invention in other aspects generally to high-quality galliumnitride (Al, Ga, In)N films and methods for making such films.

According to one implementation, a method for making a low-defectsingle-crystal gallium nitride (GaN) film is provided. An epitaxialaluminum nitride (AlN) layer is deposited on a substrate. A firstepitaxial GaN layer is grown on the AlN layer by HVPE under a growthcondition that promotes the formation of pits, wherein after growing thefirst GaN layer the GaN film surface morphology is rough and pitted. Asecond epitaxial GaN layer is grown on the first GaN layer to form a GaNfilm on the substrate. The second GaN layer is grown by HVPE under agrowth condition that promotes filling of the pits, and after growingthe second GaN layer the GaN film surface morphology is essentiallypit-free.

According to another implementation, the GaN growth condition forgrowing the first GaN layer is selected from the group consisting of ahigher growth rate than during growth of the second GaN layer, a lowergrowth temperature than during growth of the second GaN layer, a higherammonia flow than during growth of the second GaN layer, and two or moreof the foregoing.

According to another implementation, a low-defect single-crystal GaNfilm produced according to any of the above methods is provided.

According to another implementation, a low-defect single-crystal GaNfilm is provided. The GaN film has a characteristic dimension of about 2inches or greater, and a thickness normal to the characteristicdimension ranging from approximately 10 to approximately 250 microns.The GaN film includes a pit-free surface. The threading dislocationdensity on the GaN film surface being less than 1×10⁸ cm⁻².

According to another implementation, low-defect single-crystal galliumnitride (GaN) on substrate structure is provided. The structure includesa substrate, an epitaxial aluminum nitride (AlN) layer on the substrate,and a GaN film on the substrate. The GaN film includes a first epitaxialGaN growth layer and a second epitaxial GaN growth layer. The firstepitaxial GaN layer is grown on the AlN layer under a growth conditionthat promotes the formation of pits, and after growing the first GaNlayer the GaN film surface morphology is rough and pitted. The secondepitaxial GaN is grown on the first GaN layer by HVPE under a growthcondition that promotes filling of the pits formed, and after growingthe second GaN layer the GaN film surface morphology is essentiallypit-free.

According to any of the above implementations, the threading dislocationdensity on the GaN film surface is minimal. In one example, thethreading dislocation density on the surface of the GaN film may be lessthan 1×10⁸ cm⁻², in another example less than 5×10⁷ cm⁻², in anotherexample less than 1×10⁷ cm⁻², and in another example less than 5×10⁶cm⁻².

According to any of the above implementations, the amount of bowing theGaN film on an underlying substrate is minimal. In one example, the bowof the GaN film may be less than about 200 microns. In another example,the bow of the GaN film may be less than about 100 microns. In anotherexample, the bow of the GaN film may be less than about 50 microns. Inanother example, the bow of the GaN film may be less than about 25microns.

According to any of the above implementations, the surface of the GaNfilm may have a root-mean square (RMS) surface roughness of about 0.5 nmor less.

The present invention in other aspects generally relates to large-area,inclusion-free, semi-insulating gallium nitride (Al, Ga, In)N articlesand methods for growing such articles.

According to one implementation, a method is provided for making aninclusion-free uniformly semi-insulating GaN crystal. An epitaxialnitride layer is deposited on a single-crystal substrate. A 3Dnucleation GaN layer is grown on the epitaxial nitride layer by HVPEunder a substantially 3D growth mode, wherein a surface of thenucleation layer is substantially covered with pits and the aspect ratioof the pits is essentially the same. A GaN transitional layer is grownon the 3D nucleation layer by HVPE under a condition that changes thegrowth mode from the substantially 3D growth mode to a substantially 2Dgrowth mode. After growing the transitional layer, a surface of thetransitional layer is substantially pit-free. A bulk GaN layer is grownon the transitional layer by HVPE under the substantially 2D growthmode. After growing the bulk layer, a surface of the bulk layer issmooth and substantially pit-free. The GaN is doped with a transitionmetal during at least one of the foregoing GaN growth steps.

According to another implementation, an inclusion-free uniformlysemi-insulating GaN crystal is provided, which is produced according tothe foregoing method.

According to another implementation, a GaN/substrate bi-layer is formedaccording to the foregoing method. The GaN/substrate bi-layer is cooledfrom a growth temperature at which the bulk layer is grown to an ambienttemperature, wherein GaN material of the bi-layer separates from thesubstrate to form a substantially crack-free free-standing GaN article.

According to another implementation, an inclusion-free uniformlysemi-insulating free-standing GaN article is provided, which is producedaccording to the foregoing method.

According to another implementation, a method is provided for making aninclusion-free uniformly semi-insulating GaN substrate. A c-plane GaNseed substrate is provided. An epitaxial GaN boule is grown on the seedsubstrate by HVPE in a reactor. The growth mode is a substantially 2Dgrowth mode and a surface of the growing GaN boule is smooth andpit-free. A volatile iron compound is flowed into the reactor. The GaNboule is sliced into a plurality of wafer blanks. The wafer blanks arepolished to form a plurality of epi-ready GaN substrates.

According to another implementation, a semi-insulating GaN substrate isprovided, in which the sheet resistance is uniformly greater than 1×10⁵ohm/square as measured with a non-contact eddy-current based sheetresistance mapping system such as the Lehighton method.

According to another implementation, a semi-insulating GaN substrate isprovided, in which the resistivity is greater than about 1×10⁷ ohm-cm.

According to another implementation, a semi-insulating GaN substrate isprovided, in which the density of brown-spot inclusions is less thanabout 1 cm⁻².

The present invention in other aspects generally relates to high-qualitygallium nitride (Al, Ga, In)N articles that include a polycrystallineGaN layer, and to methods for growing such articles.

According to one implementation, a method is provided for making afree-standing GaN article. An epitaxial nitride layer is deposited on asingle-crystal substrate to form a nitride-coated substrate. A 3Dnucleation GaN layer is grown on the epitaxial nitride layer by HVPEunder a substantially 3D growth mode. A GaN transitional layer is grownon the 3D nucleation layer by HVPE under a condition that changes thegrowth mode from the substantially 3D growth mode to a substantially 2Dgrowth mode. A bulk GaN layer is grown on the transitional layer by HVPEunder the substantially 2D growth mode. A polycrystalline GaN layer isgrown on the bulk GaN layer.

According to another implementation, a GaN article is provided, which isproduced according to the foregoing method.

According to another implementation, the foregoing method is carried outto form a GaN/substrate bi-layer. The GaN/substrate bi-layer is cooledfrom a growth temperature at which the bulk layer is grown to an ambienttemperature, wherein GaN material of the bi-layer cracks laterally andseparates from the substrate to form a substantially crack-freefree-standing GaN article.

According to another implementation, a free-standing GaN article isprovided, which is produced according to the foregoing method.

Other aspects, features and embodiments of the invention will be morefully apparent from the ensuing disclosure and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a vertical HVPE reactor.

FIG. 2 is an optical micrograph at 50× magnification of the surface of aGaN film grown on an AlN-coated sapphire substrate under a typical HVPEGaN growth condition. The GaN film thickness was about 1 micron.

FIG. 3 is an optical micrograph of the surface of a GaN film grown on abare sapphire without any pretreatment. The GaN film thickness was about1 micron.

FIG. 4 is an optical micrograph at 50× magnification of a GaN film,about 5 microns thick, grown on an AlN-coated sapphire substrate usingan HVPE technique. Microcracking of the film is visible.

FIG. 5 is an optical micrograph (Nomarski) of ˜300 μm thick GaN filmgrown on an AlN-coated sapphire substrate using an HVPE technique underconditions for producing a smooth surface morphology. Out-of-focuscracks in the bulk of the material can be seen in the image.

FIG. 6 is an optical micrograph for a GaN film grown under a low NH₃partial pressure growth condition: a growth rate of 88 microns per hour,a growth temperature of 950° C., and a V:III ratio of 26. The filmthickness was about 6 microns.

FIG. 7 is an optical micrograph at 50× magnification of a crack-free butpitted single-crystal GaN film grown on sapphire under a moderate NH₃partial pressure growth condition: a growth rate of 320 microns perhour, a growth temperature of 990° C., and a V:III ratio of 58. The GaNfilm thickness was about 100 microns.

FIG. 8 is an optical micrograph of a polycrystalline GaN film grownunder a high NH₃ partial pressure growth condition: a growth rate of 320microns per hour, a growth temperature of 990° C., and a V:III ratio of75. The GaN film thickness was about 320 microns.

FIG. 9 is a graph showing pitting percentage, defined as the percentageof area covered with pits on GaN film, versus NH₃ flow, for growthtemperatures between 1050° C. and 1100° C. The HCl flow rate was about120 sccm, the growth rate was about 300 microns per hour, and the filmthickness was about 100 microns.

FIG. 10 a is an optical micrograph of a 2.64 mm thick HVPE GaN filmgrown under pitted growth conditions, showing the surface covered withapproximately 60% of pits.

FIG. 10 b is an optical micrograph of a 5.28 mm thick HVPE GaN filmgrown under pitted growth conditions, showing the surface ispolycrystalline.

FIG. 11 is a graph showing pit size versus growth time for severalgrowth conditions.

FIG. 12 is schematic view of a GaN crystal structure that includes abulk GaN crystal grown on a substrate, showing different layers of thecrystal structure.

FIG. 13 illustrates an evolution of GaN surface morphology during thevarious steps of bulk GaN crystal growth on a substrate according to animplementation of the present invention. The surface of the growing GaNcrystal during the bulk growth retains a pit-free smooth morphology.

FIG. 14 illustrates an evolution of GaN surface morphology during thevarious steps of bulk GaN crystal growth on a substrate according toanother implementation of the present invention. The surface of thegrowing GaN crystal during the bulk growth remains faceted.

FIG. 15 is an illustration of an example of a process for growing asemi-insulating bulk GaN crystal, including the temperature, NH₃ flows,and HCl flows for the nucleation, transition, and bulk growth stages.

FIG. 16 is an optical image of a 3″ GaN substrate or boule grown withthe semi-insulating growth scheme.

FIG. 17 is a room-temperature panchromatic cathodoluminescence image ofthe surface of a semi-insulating GaN substrate or boule. The dislocationdensity is 5×10⁴ cm⁻². The image size is approximately 118 μm×90 μm.

FIG. 18 is an x-ray rocking curve for the GaN substrate grown with thesemi-insulating growth scheme.

FIG. 19 illustrates plots of carrier concentration data (left) and Hallresistivity data (right) for a wafer created with the semi-insulatinggrowth scheme that shows high resistivity (>10⁸ ohm-cm) at roomtemperature.

FIG. 20 is a room temperature Corema resistivity map of asemi-insulating GaN substrate. The mean resistivity of the substrate is1.07×10⁹ Ω·cm.

FIG. 21 is an illustration of an example of a growth process forundoped, bulk GaN with a pitted surface morphology, including thetemperature, NH₃ flows, and HCl flows for the nucleation, transition,and bulk growth stages.

FIG. 22 is an optical image of a crack-free boule grown with the undopedgrowth scheme.

FIG. 23 is a 30×30 μm room-temperature panchromatic cathodoluminescenceimage of the surface of a polished undoped GaN boule. The dislocationdensity is 8.8×10⁶ cm⁻².

FIG. 24 is an illustration of the growth process for n-type, bulk GaNwith a smooth surface morphology, including the temperature, NH₃ flows,and HCl flows for the nucleation, transition, and bulk growth stages.

FIG. 25 is an illustration of the growth process for n-type, bulk GaNwith a pitted surface morphology, including the temperature, NH₃ flows,and HCl flows for the nucleation, transition, and bulk growth stages.

FIG. 26 illustrates plots of the calculated in-plane biaxial stresses inGaN and sapphire for a simple two-layer stress model as a function ofGaN layer thickness. Positive stress values are tensile and negativestress values are compressive. The thickness of the sapphire is 430 μmand the growth temperature is taken to be 970° C.

FIG. 27 is a schematic view of a GaN crystal structure that includes abulk GaN crystal grown on a substrate, showing different layers of thecrystal structure.

FIG. 28 is a schematic view of the GaN crystal structure illustrated inFIG. 7 after self-separation, yielding a free-standing GaN crystalseparate from the underlying substrate.

FIG. 29 is a schematic illustration of an example of a two-step GaNgrowth process of the present invention.

FIG. 30 is an optical micrograph at 200× magnification of the surface ofa GaN layer at the end of a growth step in which the surface is pitted,according to one implementation of the present invention.

FIG. 31 is a cross-sectional view of a bowed wafer or layer of material.

FIG. 32 is an optical micrograph at 50× magnification of the surface ofan as-grown 60-micron GaN film on sapphire substrate, according to oneimplementation of the present invention.

FIG. 33 is a 10×10-micron AFM scan of a 60-micron thick GaN film onsapphire, grown according to one implementation of the presentinvention.

FIG. 34 is a photograph of an iron-doped GaN wafer showing a few browninclusions.

FIG. 35 is a schematic view of a GaN crystal structure that includes abulk GaN crystal grown on a substrate, showing different layers of thecrystal structure.

FIG. 36 is a schematic view of the GaN crystal structure illustrated inFIG. 35 after self-separation, yielding a free-standing GaN crystalseparate from the underlying substrate.

FIG. 37 is a photograph of an inclusion-free semi-insulating GaN waferobtained according to one implementation of the present invention.

FIG. 38 is a photograph of 2″ freestanding substrate separated fromsapphire after cooling from the growth temperature.

FIG. 39 is a schematic illustration of a method for producing a singlecrystal GaN wafer according to an implementation of the presentinvention.

FIG. 40 is an optical micrograph of a GaN film according to animplementation of the present invention.

FIG. 41 is an illustration of a GaN growth process, including thetemperature, NH₃ flows, and HCl flows for the nucleation, transition,bulk growth, and polycrystalline growth stages according to one exampleof the present invention.

FIG. 42 is an optical micrograph of a GaN film according to animplementation of the present invention.

FIG. 43 is an illustration of a GaN growth process, including thetemperature, NH₃ flows, and HCl flows for the nucleation, transition,bulk growth, and polycrystalline growth stages according to anotherexample of the present invention.

DETAILED DESCRIPTION

Throughout the disclosure, unless specified otherwise, certain terms areused as follows. “Single crystalline film” or “single crystal” means acrystalline structure that can be characterized with x-ray rocking curvemeasurement. The narrower the peak of the rocking curve, the better thecrystal quality. “Single crystal” does not necessarily mean that thewhole crystal is a single grain; it may contain many crystalline grainswith orientation more or less aligned. “Polycrystalline film” or“polycrystal” means that a crystal has many grains whose crystalorientations are randomly distributed. An X-ray rocking curvemeasurement of a polycrystalline film does not exhibit a peak.“Microcracks” are a cluster of localized cracks with high density ofcracks. The distance between the parallel cracks in the microcrackcluster is typically less than 100 microns. “Growth cracks” are thecracks formed during crystal growth. “Cool down cracks” or “thermalcracks” are the cracks formed after the crystal growth and during thecooling of the crystal from the growth temperature to ambient or roomtemperature. “Pits” are typically inverse pyramidal pits on the crystalsurface. “Pit-free surface” is a surface essentially having no pits onits surface. “2D growth mode” means that a growth surface remains planarand smooth during the growth. “3D growth mode” means that a growthsurface develops non-planar, three-dimensional features such as pitsduring the growth. “Pitted surface morphology” means a surface having asubstantial amount of pits on its surface. “Faceted surface morphology”means that a single crystal film surface is completely covered with pitsso that the sides of the pits become the surface itself and the surfaceappears faceted. “Smooth surface morphology” means that a surface isspecular and has no visual defects (such as pits). “Nucleation layer” insome implementations may be the layer first grown on a substrate. Inother implementations, a “template layer” may be the layer first grownon a substrate. “Bulk layer” is where the majority of the crystal isgrown. “V:III ratio” in some implementations is the ratio of the ammoniaflow to the HCl flow used during a hydride vapor phase epitaxy GaNgrowth process. “Ammonia partial pressure” is calculated according tothe ammonia flow, the total gas flow into a reactor, and the reactorpressure. “Growth surface” or “growing surface” or “growth front” is thesurface of the crystal during the instance of the growth. “Front side”of a GaN substrate is the growth surface side. “Back side” of a GaNsubstrate is the side opposite to the front side. The c-plane GaN is apolar substrate, with one surface terminated with gallium (Ga-surface orGa face) and the other surface is terminated with nitrogen(nitrogen-surface or nitrogen face). In this disclosure, the“front-side” of the c-plane single crystal GaN wafer is the Ga-face.“Semi-insulating” (SI) is defined as the resistivity measured at roomtemperature equal to or greater than 1×10⁵ ohm-cm.

For purposes of the present disclosure, it will be understood that whena layer (or film, region, substrate, component, device, or the like) isreferred to as being “on” or “over” another layer, that layer may bedirectly or actually on (or over) the other layer or, alternatively,intervening layers (e.g., buffer layers, transition layers, interlayers,sacrificial layers, etch-stop layers, masks, electrodes, interconnects,contacts, or the like) may also be present. A layer that is “directlyon” another layer means that no intervening layer is present, unlessotherwise indicated. It will also be understood that when a layer isreferred to as being “on” (or “over”) another layer, that layer maycover the entire surface of the other layer or only a portion of theother layer. It will be further understood that terms such as “formedon” or “disposed on” are not intended to introduce any limitationsrelating to particular methods of material transport, deposition,fabrication, surface treatment, or physical, chemical, or ionic bondingor interaction.

Unless otherwise indicated, terms such as “gallium nitride” and “GaN”are intended to describe binary, ternary, and quaternary Group IIInitride-based compounds such as, for example, gallium nitride, indiumnitride, aluminum nitride, aluminum gallium nitride, indium galliumnitride, indium aluminum nitride, and aluminum indium gallium nitride,and alloys, mixtures, or combinations of the foregoing, with or withoutadded dopants, impurities or trace components, as well as all possiblecrystalline structures and morphologies, and any derivatives or modifiedcompositions of the foregoing. Unless otherwise indicated, no limitationis placed on the stoichiometries of these compounds. Thus, the term“GaN” encompasses Group III nitrides and nitride alloys; that is,Al_(x)Ga_(y)In_(z)N (x+y+z=1, 0≦x≦1, 0≦y≦1, 0≦z≦1), or (Al, Ga, In)N.

Large-area GaN substrates can be made by growing thick GaN films onforeign substrates, followed by separation of the film from thesubstrate. Prior art in U.S. Pat. Nos. 6,413,627; 6,468,347; 6,667,184;6,693,021; 6,773,504; and 6,909,165 teaches methods for making a bulkGaN substrate by first growing a GaN film on a gallium arsenide (GaAs)substrate by hydride vapor phase epitaxy (HVPE) and subsequentlyremoving the GaAs substrate by etching or grinding. Prior art in U.S.Pat. Nos. 6,440,823; 6,528,394; 6,596,079; 6,652,648; 6,750,121;6,765,240; 6,923,859; and U.S. Pat. App. Pub. Nos. 2002/0068201;2005/0103257; and 2005/0009310 teaches methods for making a bulk GaNsubstrate by first growing a GaN film on a sapphire substrate, followedby separating the grown GaN from the sapphire by, for example,laser-induced separation.

Single-crystal GaN films can be grown on sapphire substrates withvarious vapor phase growth techniques, such as molecular beam epitaxy(MBE), metal-organic vapor phase epitaxy (MOVPE), and hydride vaporphase epitaxy (HVPE). In the MBE and MOVPE growth of GaN films onsapphire, a low-temperature buffer layer is typically needed to growhigh-quality GaN films. It is not clear whether a buffer layer is neededfor HVPE GaN growth on sapphire. Lee in U.S. Pat. No. 6,528,394discloses a specific method of pre-treatment for growing GaN on sapphireusing HVPE. The pre-treatment involves etching sapphire with a gasmixture of hydrochloric acid (HCl) and ammonia (NH₃), as well asnitridation of the sapphire substrate. Molnar in U.S. Pat. No. 6,086,673discloses the use of a zinc oxide (ZnO) pretreatment layer that wasfurther reacted in the gaseous environment of HCl and/or NH₃. After thistreatment of the sapphire substrate, single-crystal GaN film is thengrown by HVPE. On the other hand, Vaudo et al. in U.S. Pat. No.6,440,823 discloses the growth of a low defect density GaN layer onsapphire by an HVPE method, without using any buffer layers ornucleation layers.

Since teachings in the prior art regarding sapphire substrate treatmentor initiation prior to HVPE GaN growth are in conflict, wesystematically investigated the growth of gallium nitride film onsapphire using an HVPE process. Vertical HVPE reactors were used for theinvestigation. FIG. 1 schematically illustrates an example of a verticalHVPE reactor 100. The HVPE reactor 100 includes a quartz reactor tube104 that is heated by a multi-zone furnace 108. The reactor tube 104 isconnected to gas inlets 112, 116, and 120 for introducing reactants,carrier gases, and diluting gases. The reactor tube 104 is alsoconnected to a pump and exhaust system 124. In some implementations,inside the reactor 100, gaseous hydrochloric acid (HCl) is flowedthrough a vessel 128 containing gallium metal 132, which is at atemperature of, for example, about 850° C. The hydrochloric acid reactswith the gallium metal 132, forming gaseous GaCl, which is transportedby a carrier gas, such as nitrogen, to the deposition zone in thereactor tube 104. Ammonia (NH₃) and an inert diluent gas, such asnitrogen, are also flowed to the deposition zone where GaN crystals aredeposited. The reactor 100 is designed such that the mixing of GaCl andNH₃ does not occur near the gas outlets, ensuring no deposition of GaNon the outlets of GaCl and NH₃ and enabling long-term stability of gasflow patterns. Epi-ready c-plane sapphire substrates or other suitablesubstrates 136 may be used. The substrate 136 is placed on a rotatingplatter 140 and heated to a temperature of, for example, 900-1100° C.

A typical deposition run process is as follows: (1) a substrate 136 isplaced on the platter 140, (2) the reactor 100 is sealed, (3) thereactor 100 is evacuated and purged with high-purity nitrogen to removeany impurities from the system, (4) the platter 140 with the substrate136 is raised to the deposition zone, (5) the platter temperature iscontrolled at the desired deposition temperature, (6) ammonia is flowedinto the reactor 100, (7) HCl is flowed to the reactor 100 to start theGaN deposition, (8) deposition proceeds according to a predeterminedrecipe for a predetermined time, (9) the HCl and NH₃ gas flows arestopped, (10) the platter 140 is lowered and the grown crystal isgradually cooled down, and (11) the grown crystal is removed forcharacterization and further processing.

After systematically investigating the HVPE growth of GaN on sapphiresubstrates, we uncovered several issues that were not disclosed in theprior art, namely, (1) irreproducible nucleation of single-crystal GaNfilm on untreated sapphire substrate, (2) microcracking ofsingle-crystalline GaN films, (3) fracture of the sapphire substrateduring deposition of thick GaN crystals, and (4) instability of surfacemorphology during long GaN boule growth.

GaN Nucleation on Sapphire

First, we grew many HVPE GaN films directly on sapphire substrateswithout any buffer layer or pretreatment under the conditions taught bythe prior art, i.e., growth temperature about 950-1050° C., V:III ratio(i.e., NH₃/HCl) of about 10-50, and growth rate of about 100 microns perhour. The bare sapphire substrate was heated up to the growthtemperature, ammonia flow was turned on first to fill the reactor to apre-determined partial pressure, and HCl flow was turned on to initiatethe growth. After analyzing the grown GaN films with x-ray rocking curveand optical microscope, we determined that the GaN films grown directlyon bare sapphire substrates were not single-crystalline films. In fact,they were polycrystalline GaN. We wish not to be bound by any particulartheory regarding the various results of HVPE GaN crystal growth onsapphire, but the discrepancy in the various prior-art work and our ownwork may be related to particular reactor configurations or surfacetreatments. The prior art did not teach a reproducible method forgrowing single-crystal GaN films on sapphire substrates by HVPE.

There is a large lattice mismatch between sapphire and gallium nitride.Furthermore, c-plane GaN is a polar crystal, i.e., one face isterminated with gallium and the opposite face of the crystal isterminated with nitrogen. On the other hand, sapphire is not a polarcrystal; the c-plane of sapphire is terminated with oxygen on bothfaces. In other GaN thin-film deposition techniques such as molecularbeam epitaxy (MBE) or metal-organic vapor phase epitaxy (MOVPE), a thinbuffer layer is required for high-quality single-crystalline GaN growth.The buffer layer may be an AlN layer (S. Yoshida et al., Appl. Phys.Lett., 42, 427 (1983); H. Amano et al, Appl. Phys. Lett. 48, 353 (1986))or a GaN layer grown at low temperature (S. Nakamura, Jpn. J. Appl.Phys. 30. L1705 (1991)). Lee in U.S. Pat. No. 6,528,394 postulatedformation of thin AlN layer on the sapphire surface by the pre-treatmentstep prior to HVPE GaN growth.

U.S. Pat. No. 6,784,085, the entire contents of which are incorporatedby reference into the present disclosure, discloses a high-temperaturereactive sputtering method for growing high-quality AlN film on sapphiresubstrate. Using this method, we coated sapphire substrates with AlN foruse as substrates for HVPE GaN growth. High-quality GaN thin films weresuccessfully and reproducibly grown on the AlN-coated sapphiresubstrate. The following examples compare HVPE GaN growth on baresapphire substrates and on AlN-coated sapphire substrates.

EXAMPLE 1 Single Crystal GaN Film Grown on AlN-coated Sapphire

In this example, we used a 2″ sapphire substrate. A thin layer of AlNfilm was sputter-grown on the sapphire substrate using the methoddisclosed in U.S. Pat. No. 6,784,085. The thickness of the AlN layer wasabout 0.5-1.5 microns. X-ray rocking curve measurement indicated the AlNfilm was epitaxial single-crystalline film with (0002) rocking curvefull width at half maximum (FWHM) of 50 arcsec. The AlN-coated sapphiresubstrate was loaded into the HVPE reactor and a GaN film was grownusing the aforementioned procedure. The growth rate was about 60 micronsper hour, the GaCl partial pressure was about 3 (2.97) Torr, the NH₃partial pressure was about 45 (44.6) Torr, the V:III ratio was about 15,and the growth temperature was about 950° C. as measured with athermocouple under the platter. The growth time was 1 minute. The GaNfilm grown was transparent with a smooth specular surface. FIG. 2 showsan optical micrograph of the surface of the GaN film. FIG. 2 showstypical hillock surface morphology for HVPE GaN film. X-ray rockingcurve measurement confirmed the single-crystalline nature of the GaNfilm, with a FWHM value of 297 arcsec.

COMPARATIVE EXAMPLE 1 Polycrystalline GaN Film Grown on Bare Sapphire

In this comparative example, we used a bare 2″ sapphire substrate. Thebare sapphire substrate was loaded into the HVPE reactor, and a GaN filmwas grown using the aforementioned procedure. No particularpre-treatment was performed on the sapphire substrate. The growthconditions were the same as the conditions used in Example 1: the growthrate was about 60 microns per hour, the GaCl partial pressure was about3 Torr, the NH₃ partial pressure was about 45 Torr, the V:III ratio wasabout 15, and the growth temperature was about 950° C. The growth timewas 1 minute. The GaN film was frost-like and not transparent. FIG. 3shows an optical micrograph of the surface of the GaN film growndirectly on the bare sapphire substrate. The surface is very rough,showing a multi-grained thin film. X-ray rocking curve measurementdemonstrated that the GaN film grown on bare sapphire waspolycrystalline.

GaN Film Microcracking

After developing the nucleation of a GaN single-crystalline film on AlNsputter-coated sapphire substrates, we investigated the growth ofthicker GaN films. We discovered a problem, namely, microcracking in theGaN films. The HVPE growth conditions were chosen to produce a smoothGaN surface. FIG. 4 shows an optical micrograph of thin GaN film about 5microns thick on an AlN-coated sapphire substrate. The surface exhibitstypical smooth HVPE GaN morphology with hillock features. However,microcracks in the GaN film are apparent. The sapphire substrate remainsintact without any cracking in this case. Microcracking also occurs inthicker GaN films. For thicker films (˜300-1000 μm) cracks can be seenintersecting the surface and below the surface of the film (FIG. 5). Byfocusing the microscope into the bulk of the film, different strata ofcracks can be seen down to the sapphire substrate. The sapphiresubstrates were also cracked when the thick GaN films were grown.

Because of the difference between the coefficients of thermal expansionof the sapphire substrate and the GaN film, thermal stress builds upwhen the film cools down from the typical growth temperature of about1000° C. to ambient room temperature. As discussed in open literature(for example, E. V. Etzkom and D. R. Clarke, “Cracking of GaN Films,” J.Appl. Phys., 89 (2001) 1025), sapphire substrate shrinks faster than GaNfilm during cool down, causing a compressive stress in the GaN film dueto this thermal expansion mismatch. The compressive thermal stress inthe GaN film should not cause microcracking in the GaN film during cooldown. Therefore, the microcracks must be formed during the GaN growthand prior to cool down.

The microcracking of the GaN film during the growth suggests a tensilestress in the GaN film during the growth. We wish not be bound by anyparticular theory regarding the origin of microcracking during GaNgrowth. However, the tensile stress may be related to the AlN layeremployed in the study, or may be related to the HVPE growth conditionused, or may be universal to the HVPE GaN growth in general. Whilecracking is noted in some instances, most prior-art teachings in HVPEGaN growth do not disclose the formation of microcracks in GaN filmduring growth. The prior art also does not teach how to eliminate themicrocracks during the HVPE GaN growth.

In order to eliminate the microcracks formed during the HVPE GaN growth,we systematically investigated GaN growth on the AlN-coated sapphireunder various growth conditions by varying growth parameters such asGaCl flow or partial pressure (which may be determined by the flow ofhydrochloric acid (HCl)), NH₃ flow or partial pressure, growthtemperature, and associated variables such as growth rate and V:IIIratio (e.g., NH₃/HCl ratio). In this example, the V:III ratio is theratio of the NH₃/HCl flow. The growth rate is typically proportional toGaCl partial pressure, which is directly related to the HCl flow. Wefound that the surface morphology varies substantially with the growthtemperature, growth rate, and ammonia partial pressure (or V:III ratio).If the “optimal” HVPE GaN growth condition is defined as the conditionthat yields a smooth, crack-free, thin GaN film (e.g., equal to or lessthan 3 microns thick) on the AlN-coated sapphire substrate, a thick GaNfilm (e.g., equal to or greater than 20 microns thick) grown under such“optimal” condition is microcracked. The GaN film, grown under acondition with higher growth temperature and/or lower V:III ratio thanthe “optimal” growth condition, has a higher density of cracking. On theother hand, the GaN film, grown under a condition with slightly lowergrowth temperature and/or higher V:III ratio, is crack-free but withmacroscopic pits on the surface.

At a constant growth temperature and GaCl partial pressure, increasingNH₃ partial pressure dramatically alters the behavior of microcrackingand surface morphology. For a constant growth time (similar filmthickness, about 100 microns), the HVPE GaN surface morphology graduallychanges from a smooth, hillocked morphology with microcracks at low NH₃partial pressure, to a surface covered with pits at moderately high NH₃partial pressure, and eventually to polycrystalline material at high NH₃partial pressure. When the GaN film is covered with pits, microcrackingduring the growth may be eliminated. The following Examples 2-4illustrate the effect of NH₃ partial pressure (V:III ratio) on thesurface morphology.

EXAMPLE 2 GaN Growth Under Low NH₃ Partial Pressure Conditions

In this example, we grew a GaN film under a low NH₃ partial pressurecondition (low V:III ratio). An AlN-coated 2″ sapphire substrate wasused. The AlN coated sapphire substrate was loaded into the HVPE reactorand a GaN film was grown using the aforementioned procedure. The growthrate was about 87.5 microns per hour, the GaCl partial pressure wasabout 0.73 Torr, the NH₃ partial pressure was about 19.5 Torr, the V:IIIratio was about 26, and the growth temperature was about 950° C. Thegrowth time was 5 minutes. The GaN film grown was transparent with asmooth surface. However, the GaN film was microcracked. FIG. 6 shows anoptical micrograph of the surface of the GaN film, showing microcrackingof the film. X-ray rocking curve measurement confirmed the singlecrystalline nature of the GaN film, with FWHM of 300 arcsec. Theobserved trend of microcracked films under low NH₃ partial pressureconditions was consistent over a range of growth rates and temperatures.The partial pressure of NH₃ required to prevent microcracking increasedwith increasing temperature.

EXAMPLE 3 GaN Growth Under Moderate NH₃ Partial Pressure Conditions

In this example, we grew a GaN film under a moderate NH₃ partialpressure condition (moderate V:III ratio). An AlN-coated 2″ sapphiresubstrate was used. The AlN coated sapphire substrate was loaded intothe HVPE reactor, and a GaN film was grown using the aforementionedprocedure. The growth rate was about 320 microns per hour, the GaClpartial pressure was around 1.8 Torr, the NH₃ partial pressure wasaround 112.8 Torr, the V:III ratio was about 58, and the growthtemperature was about 990° C. The growth time was 20 minutes. Underoptical microscope examination, the GaN surface was covered with pits.FIG. 7 shows an optical micrograph of the surface of the GaN film.Although the GaN film surface is covered with pits, the film is still anepitaxial single-crystalline film, as confirmed by x-ray rocking curvemeasurement, with FWHM of 400 arcsec. The larger FWHM value of the filmis due in part to curvature of the sample, which is known to broaden theX-ray diffraction peak.

EXAMPLE 4 GaN Growth Under High NH₃ Partial Pressure Conditions

In this example, we grew a GaN film under a high NH₃ partial pressurecondition (high V:III ratio). An AlN-coated 2″ sapphire substrate wasused. The AlN coated sapphire substrate was loaded into the HVPEreactor, and a GaN film was grown using the aforementioned procedure.The growth rate was about 320 microns per hour, the GaCl partialpressure was around 1.8 Torr, the NH₃ partial pressure was around 135Torr, the V:III ratio was about 75, and the growth temperature was about990° C. The growth time was 60 minutes. The GaN film grown waslight-brown with a rough surface. FIG. 8 shows an optical micrograph ofthe surface of the GaN film, showing many grains. X-ray rocking curvemeasurement confirmed the film was polycrystalline.

Similar surface morphology trends are observed with growth temperatureat otherwise constant conditions, or with growth rate at otherwiseconstant conditions. Under constant GaCl and NH₃ partial pressures(constant growth rate and V:III ratio), reducing the growth temperaturealters the growth morphology from a smooth, hillocked structure to apitted surface morphology and eventually to polycrystalline growth.Similarly, at a constant growth temperature and V:III ratio, increasingthe growth rate (by increasing both GaCl and NH₃ partial pressure)alters the surface morphology from a smooth pit-free surface to a pittedsurface and eventually to a polycrystalline morphology.

Since the pitted surface morphology eliminates the microcracks in theGaN film during the growth, we extensively studied the growth conditionsthat could yield pitted morphology. Furthermore we defined a pittingpercentage as the percentage of the surface area covered with pits.Typically, for a 100-micron thick GaN film grown under constantconditions on AlN-coated sapphire substrate, a pitting percentagegreater than 30% in the GaN film eliminates the growth microcracks. Weevaluated pitting percentage as a function of growth NH₃ partialpressure for several growth temperatures. FIG. 9 illustrates pittingpercentage for 100-micron thick GaN films grown on AlN-coated sapphiresubstrates versus the NH₃ flow for furnace temperatures ranging from1050° C. to 1100° C. The growth rate for this study was about 300microns per hour and the film thickness was about 100 microns. At lowergrowth temperature, a slight change in NH₃ partial pressure leads to alarge change in pitting percentage, whereas at higher growthtemperature, a change in NH₃ partial pressure leads to a lesser changein pitting percentage. This indicates that growth morphology is moresensitive to the NH₃ partial pressure at lower temperature than athigher growth temperature.

Thick GaN Film GaN Growth

After discovering that HVPE GaN grown under conditions that yield pittedsurface morphology eliminates the microcracks in GaN during the growth,we extended the growth time to grow thick films. However, we found thatfor most pitted growth conditions, extending the growth time leads to agradual formation of polycrystalline material. FIG. 10 a is opticalmicrograph of a 2.64 mm (2,640 micron) thick GaN film grown under apitted growth condition (growth rate 110 microns per hour, growth time24 hours, growth temperature around 940° C., and V:III ratio around 48).The HVPE GaN film is covered with approximately 60% of pits. FIG. 10 bshows an optical micrograph of a GaN film grown under conditionsidentical to the film shown in FIG. 10 a, but 24 hours longer. Thesurface material is polycrystalline material. A cross-sectionalexamination under optical microscope suggested that the GaN growthsurface nucleated polycrystalline material at about 30 hours aftergrowth. The prior art did not teach the growth morphology instabilitywith longer growth and how to overcome it.

We developed several strategies to overcome the deterioration of GaNsurface morphology during long HVPE GaN growth. We found that the sizeof the pits typically grows with the GaN growth time, and that the GaNgrowth surface eventually nucleates polycrystalline material. Sincethere are broad growth parameter (growth rate, growth temperature,ammonia partial pressure, and V:III ratio) windows that produce a pittedsurface morphology, there may be conditions where the pit size does notgrow with GaN growth time. We have systematically evaluated GaN growthwith the pitted surface morphology. It is noted that the size of thevarious pits on the surface is not the same, and typically follows anormal distribution curve. Sometimes there are more than one pit sizedistribution for a film, for example, a distribution curve for smallerpits and a distribution curve for larger pits.

The experiments were performed in the following way. In the first stageof the growth, a pitted GaN layer was grown on AlN-coated sapphiresubstrate under the pitted condition for 1 to 4 hrs, depending on thegrowth rate. Then the growth conditions were changed at the secondstage, and growth was allowed to proceed for additional time. The pitsize and the growth rate of the pit size were analyzed. We found thatpits formed in the first stage evolve during the second stage accordingto the growth condition of the second stage. The pits can be completelyfilled to form a pit-free GaN film in the second stage at the conditionsof either higher growth temperature, or lower growth rate, or lowerammonia partial pressure, or a combination thereof. The pit size can bemaintained or the pit size can be increased if the growth condition ofthe second stage is close to the condition of the first stage.

FIG. 11 shows pit size versus growth time for several growth conditions.Under a growth condition of growth rate of 320 microns per hour, V:IIIratio of around 10, and growth temperature of 1000° C., the size of thepits was maintained or decreased slightly with longer growth time. Underanother condition of growth rate of 320 microns per hour, V:III ratio ofaround 12, and growth temperature of 1000° C., the pit size increaseslinearly with growth time. The growth conditions discussed here arespecific to the HVPE reactor used. The exact condition may differ if adifferent type of reactor configuration is used. However, the generaltrend should be similar.

Bulk GaN Crystal Growth

According to implementations of the invention, several methods forgrowing bulk GaN crystals on appropriate substrates will now bedescribed. Generally, the methods involve several aforementioned aspectsof HVPE GaN growth that were not taught in the prior art. First, toreproducibly nucleate GaN single crystalline film, certainimplementations utilize a thin, epitaxial single-crystalline nitridelayer (e.g., AlN) formed by high-temperature sputter growth. Second, toprevent microcracking of GaN film during HVPE growth, growth conditionsare utilized that produce a pitted surface morphology. Third, multiplesteps with different growth conditions are utilized to control thesurface morphology during bulk crystal growth.

Bulk GaN crystal growth in some implementations generally includes threemajor stages: nucleation with a pitted surface morphology on anitride-coated substrate or a bare substrate, transitioning the growthcondition from the nucleation stage to the bulk growth stage, and bulkgrowth. Each stage may include one or more steps. The growth parametersin each step may be held constant or may be ramped or stepped.

FIG. 12 illustrates an example of a bulk crystal structure 1200 producedaccording to methods described in the present disclosure. The bulkcrystal structure 1200 includes a substrate 1204, an epitaxial nitridelayer 1208 such as a Group III nitride (e.g., AlN), a nucleation layer1212 of GaN, a GaN transitional layer 1216, and a bulk GaN layer orcrystal 1220. These different GaN layers 1212, 1216 and 1220 may begrown under different growth conditions. The purpose of the nucleationGaN layer 1212 is to prevent microcracking during the growth, and thepurpose of the transitional GaN layer 1216 is to provide a stablesurface morphology for the growing crystal for the bulk GaN crystalgrowth. The transition between different GaN layers 1212, 1216 and 1220may be gradual, i.e., no distinct interface may exist between differentGaN layers 1212, 1216 and 1220.

In some implementations, the substrate 1204 may have a characteristicdimension (e.g., diameter) of about 2 inches or greater. As furtherexamples, the substrate 1204 may be about 3″ or greater, about 4″ orgreater, or any other suitable size such as about 12″ or greater. Thesubstrate 1204 may be sapphire (Al₂O₃), although other suitablesingle-crystal substrates 1204 may be utilized. Non-limiting examples ofsuitable substrates 1204 include sapphire, silicon carbide, galliumarsenide, zinc oxide, silicon, spinel, lithium gallate, lithiumaluminate, etc.

An epitaxial nitride thin film 1208 such as AlN may be deposited on thesubstrate 1204 with a suitable technique such as high-temperaturesputtering, metal-organic vapor phase deposition, molecular beamepitaxy, hydride vapor phase epitaxy, or high-temperature annealing ofthe substrate 1204 in ammonia. In one example, the thickness of the AlNlayer 1208 is in the range (ranges) from about 0.05 to about 10 microns.In another example, the thickness of the AlN layer 1208 is in the rangefrom about 0.5 to 2 microns. In another example, the thickness of theAlN layer 1208 is in the range from about 0.5 to 1.5 microns. The AlNlayer 1208 serves as a template for single-crystalline GaN film growth.

The purpose of the GaN nucleation layer 1212 is to prevent microcrackingduring subsequent growth. The thickness of GaN film 1212 grown duringthe nucleation stage may range from about 5 to about 100 microns. Inother examples, the thickness ranges from about 10 to about 50 microns.The pitting percentage of the nucleation layer 1212, defined as thepercentage of the surface covered with pits, may be greater than about50%. In other examples, the pitting percentage is greater than about75%. In other examples, the pitting percentage is greater than about90%. The diameter of the pits in the nucleation layer 1212 may rangefrom about 5 to about 50 microns. In other examples, the diameter of thepits ranges from about 10 to about 30 microns. The growth conditions forthe nucleation stage are the conditions that preferentially produce apitted surface morphology. Such conditions are, for example, highergrowth rate, and/or higher ammonia flow, and/or lower growth temperaturethan the “optimal” thin-film growth condition that would produce smooth,substantially pit-free, crack-free thin films (e.g., with a thicknessequal to or less than 3 microns), but would produce microcracked thickfilms (e.g., with a thickness equal to or greater than 20 microns).Example 1, given above, is an example of an optimized growth conditionfor a thin film. When growing a thin film (≦3 μm), this optimal”thin-film growth condition typically produces a crack-free film, whereaswhen growing a thick film (≧20 μm), the optimal” growth conditiontypically produces a microcracked film.

During the growth of the nucleation layer, the surface morphology of thegrowing GaN crystal is constantly evolving. For instance, the size ofthe pits may be increasing. For a given growth temperature, there is apreferred growth rate, ammonia partial pressure, V:III ratio andthickness to achieve the desired result. As an example, for a growthtemperature of 940° C., the growth rate for nucleation may beapproximately 190-200 μm/hr at a V:III ratio of around 17-18. In thiscase, a thickness of 15-20 μm has been shown to result in the desiredoutcome. As another example, for a growth temperature of 980° C., thegrowth rate for nucleation may be approximately 250 μm/hr at a V:IIIratio of about 50, for which a thickness of 25-30 μm has shown similarresults. If the GaN film continues to grow under the nucleation growthcondition, the GaN film will eventually become polycrystalline. Thespecific conditions for the optimal nucleation may be different if adifferent reactor geometry is employed.

In one implementation, the growth temperature in the nucleation stageranges from about 900° C. to about 1000° C., the V:III ratio in thenucleation stage ranges from about 10 to about 100, and the growth ratein the nucleation stage ranges from about 50 μm/hr to about 500 μm/hr.In some implementations, one or more of these growth conditions remainconstant or substantially constant, at a value or values of acorresponding one of the foregoing ranges, during the nucleation stage.In other implementations, one or more of these growth conditions isvaried (e.g., stepped or ramped) within values of a corresponding one ofthe foregoing ranges during the nucleation stage.

The purpose of the GaN transitional layer 1216 is to change the growthmorphology from a highly pitted nucleation to a morphology that isstable with growth time. The transitional layer 1216 may be grown undera different growth condition that gradually changes the surfacemorphology of the growing GaN crystal from a much pitted surface to aless pitted surface. The stable morphology may be pit-free texturedmorphology, or may still be a pitted surface with growth conditions thatdo not increase the size of pit with longer growth, or may be a mostlyfaceted surface wherein the facets are the sidewalls of the pits. Insome implementations, the density of the pits is gradually reducedduring the growth of the transitional GaN layer 1216. In one example,the thickness of the transitional layer 1216 may range from about 50 toabout 1000 microns (or about 0.05 to about 1 mm).

In one implementation, the surface morphology of the growing GaN crystalduring growth of the transitional layer 1216 is evolved from a muchpitted surface of the nucleation layer 1212 to a substantially pit-freesurface. This evolution of the surface morphology of the growing GaNcrystal during growth of the transitional GaN layer 1216 is accomplishedby changing the growth conditions from a pitted growth condition to aless pitted growth condition. This can be achieved by increasing thegrowth temperature, and/or by reducing NH₃ partial pressure, and/or byreducing the growth rate. The growth parameters during the transitionlayer growth may be stepped or ramped. The morphology of the growingcrystal is evolving during the growth of the transitional layer 1216.

In one implementation, the growth temperature in the transitional stageranges from about 920° C. to about 1100° C., the V:III ratio in thetransitional stage ranges from about 8 to about 80, and the growth ratein the transitional stage ranges from about 50 μm/hr to about 550 μm/hr.In some implementations, one or more of these growth conditions remainconstant or substantially constant, at a value or values of acorresponding one of the foregoing ranges, during the transitionalstage. In other implementations, one or more of these growth conditionsis varied (e.g., stepped or ramped) within values of a corresponding oneof the foregoing ranges during the transitional stage.

The bulk growth layer 1220 is grown on the transitional layer 1216, andis where the most GaN crystal is grown. In some implementations, thesurface morphology of the growing GaN crystal does not changesubstantially during the bulk growth step. The growth condition ischosen based on the criteria that GaN surface morphology does notdeteriorate during the long growth. In some examples, the bulk growthlayer 1220 is thicker than about 1 mm. In other examples, the bulkgrowth layer 1220 is thicker than about 2 mm. In other examples, thebulk growth layer 1220 is thicker than about 5 mm. In other examples,the bulk growth layer 1220 is thicker than about 1 cm. In otherexamples, the bulk growth layer 1220 is thicker than about 2 cm. Inother examples, the bulk growth layer 1220 is thicker than about 5 cm.The growth conditions may be kept constant during the bulk growth stage.Alternatively, the growth conditions may be slightly ramped in order tocompensate for any local growth chemistry change associated withparasitic GaN deposition on the reactor wall, temperature changes, orother changes in the growth environment during growth.

In some implementations, the surface morphology of the bulk layer 1220during the bulk growth stage may be smooth and essentially pit-free. Thetransition layer 1216 in this case completely eliminates the pits. Thebulk growth conditions chosen maintain such morphology throughout thebulk layer growth.

FIG. 13 illustrates an evolution of GaN surface morphology(cross-section) during the various steps of GaN bulk crystal growth onthe substrate 1204 according to this embodiment. After the growth of thenucleation GaN layer 1212, the surface 1312 of the nucleation GaN layer1212 is pitted. During the growth of the transitional GaN layer 1216,the pit size and pit density on the growing GaN crystal surface isgradually reduced. After the growth of the transitional layer 1216, thepits are eliminated and a pit-free surface 1316 is formed. During thebulk growth step, this pit-free surface morphology is maintained, asillustrated by the pit-free surface 1320 of the bulk layer 1220 in thisexample.

In other implementations, the surface morphology of the bulk layer 1220during growth may have some pits. The growth conditions may be chosen tomaintain the pit size throughout the bulk layer growth.

In other implementations, the surface morphology of the bulk layer 1220during the growth may be completely faceted. These facets are formed viathe growth and coalescence of pits during the transitional layer growth.In this case, the surface morphology of the growing GaN crystal duringthe transitional layer growth is evolved from the nucleation layergrowth with a high density of small pits to a surface with a smallerdensity of the larger pits. At the end of the transitional layer growth,the surface may be almost completely covered with the pits. The surfacemostly comprises various facets of the pits, and this surface is calleda faceted surface. The facets of the pits typically have crystal planesof (10-11), (10-12), (11-21), (11-22) and other planes. During the bulkgrowth, these facets are maintained. The growth conditions chosenprevent nucleation of polycrystalline grains at the bottom of the pits,and also maintain the faceted surface morphology.

FIG. 14 illustrates the evolution of the surface morphology during thegrowth according to this example. After the growth of the nucleationlayer 1212, the surface 1312 is covered with high density of small pits.During the growth of the transitional GaN layer 1216, some small pitsare gradually filled, some pits coalesce, and some pits grow larger.After the growth of the transitional layer 1216, the pits are coalescedand grown, forming a faceted surface morphology 1416. During the bulkgrowth, the faceted surface morphology is maintained, as illustrated bythe faceted surface 1420 of the bulk layer 1220 in this example. Thespacing between facets 1424, defined as the distance between ridges1428, may be less than 2 mm. In other examples, the spacing is less than1 mm. In other examples, the spacing is in the range from about 0.5 toabout 2 mm.

In one implementation, the growth temperature in the bulk growth stageranges from about 950° C. to about 1100° C., the V:III ratio in the bulkgrowth stage ranges from about 5 to about 50, and the growth rate in thebulk growth stage ranges from about 50 μm/hr to about 500 μm/hr. In someimplementations, one or more of these growth conditions remain constantor substantially constant, at a value or values of a corresponding oneof the foregoing ranges, during the bulk growth stage. In otherimplementations, one or more of these growth conditions is varied (e.g.,stepped or ramped) within values of a corresponding one of the foregoingranges during the bulk growth stage.

In some implementations, during the bulk growth, no impurity isintroduced and all the gas sources are purified in order to achievehigh-purity GaN crystals. In some examples, the impurity concentrationin the high-purity GaN layer 1220 is less than about 10¹⁷ cm⁻³. In otherexamples, the impurity concentration is less than about 10¹⁶ cm⁻³. Inother examples, the impurity concentration is less than about 10¹⁵ cm⁻³.

Alternatively, in other implementations, during the bulk growth, n-typeimpurities, such as silicon (introduced, for example, as silane) oroxygen, are introduced to produce n-type GaN crystals. The introductionof impurities may occur at any stage of the GaN growth. It will beunderstood that the growth conditions may be slightly different whenn-type doping is introduced. In some examples, the electronconcentration in the n-type bulk GaN layer 1220 is greater than about10¹⁷ cm⁻³. In other examples, the electron concentration is greater thanabout 10¹⁸ cm⁻³. In other examples, the electron concentration isgreater than about 10¹⁹ cm⁻³. In some examples, the resistivity of then-type GaN layer 1220 is less than about 0.1 ohm-cm. In other examples,the resistivity is less than about 0.01 ohm-cm. In other examples, theresistivity is less than about 0.001 ohm-cm.

In other implementations, the bulk GaN crystal may be p-type doped byintroducing p-type impurities such as magnesium (Mg). Mg may beintroduced as a metal-organic compound. Mg may also be introduced as Mgvapor by heating Mg metal in a quartz enclosure and carrying the Mg tothe deposition zone by carrier gas. It will be understood that thegrowth conditions may be slightly different when p-type doping isintroduced. In some examples, the hole concentration in the p-type bulkGaN layer 1220 is greater than about 10¹⁷ cm⁻³. In other examples, thehole concentration is greater than about 10¹⁸ cm⁻³. In other examples,the hole concentration is greater than about 10¹⁹ cm⁻³. In someexamples, the resistivity of the p-type GaN layer 1220 is less thanabout 0.1 ohm-cm. In other examples, the resistivity is less than about0.01 ohm-cm. In other examples, the resistivity is less than about 0.001ohm-cm.

In other implementations, the bulk GaN crystal can also be made into asemi-insulating (SI) material by introducing a deep-level acceptor.Transition metals, such as iron, cobalt, nickel, manganese, and zinc,are deep-level acceptors. The transition metal may be introduced to thedeposition zone either via a metal-organic or metal-chloride compoundformed by reacting, for example, iron metal with hydrochloric acid. Wheniron is used as a deep-level acceptor, gaseous ferrocene may beintroduced into the HVPE reactor. It will be understood that the growthconditions may be slightly different when transitional metal doping isintroduced. The concentration of the transition metal in the bulk GaNlayer 1220 may range from about 10¹⁶ to about 10²⁰ cm⁻³. In otherexamples, the concentration of the transition metal ranges from about10¹⁷ to about 10¹⁹. In other examples, the concentration of thetransition metal is around 10¹⁸ cm⁻³. The resistivity of the SI bulk GaNlayer 1220 at room temperature may be greater than about 10⁶ ohm-cm. Inother examples, the resistivity is greater than about 10⁷ ohm-cm. Inother examples, the resistivity is greater than about 10⁸ ohm-cm.

After the growth of bulk GaN crystal on the substrate 1204, the crystalmay be cooled to ambient temperature. The bulk crystal may be ground toform a desired size such as about 2 inches or greater, or in somespecific examples, about 2″, about 3″, about 4″, about 6″, about 8″, orabout 12″ with major and minor flats. Subsequently, the ground boule maybe sliced into multiple wafers via a wire saw or other suitabletechnique. The substrate 1204 and the nucleation layer GaN material 1212may be discarded. In some examples, the substrate 1204 delaminates fromthe bulk GaN crystal. Whether the substrate 1204 is attached to the bulkcrystal or not, the wafering process may be the same.

Prior to slicing the bulk GaN crystal into multiple wafers, the crystalmay be orientated into a specific crystallographic orientation. The GaNwafers may be a vicinal plane of a regular crystallographic plane. TheGaN wafers may be, for example, C-plane, or A-plane, or M-plane, or(10-1n) (n=1, 2, 3, 4, 5) family of planes, or (11-2n) (n=1, 2, 3, 4, 5)family of planes. Because of the rotational symmetry of the GaN crystal,each (10-1n) plane and (11-2n) plane represent a group of six similarplanes.

The sliced GaN wafers may be mechanically polished to a specified waferthickness. To remove the subsurface damage, the wafer may be chemicallymechanically polished as the last step. Reactive ion etching orinductively coupled plasma etching may also be used to remove thedamaged surface layer. Other suitable surface finishing techniques mayalternatively or additionally be employed.

The GaN wafers produced according to embodiments of the presentinvention are of high crystalline quality. Crystal quality can bemeasured with x-ray diffraction such as rocking curve measurement. Thefull width at half maximum (FWHM) for the GaN(0002) reflection may beless than 300 arcsec. In other examples, the FWHM is less than 200arcsec. In other examples, the FWHM is less than 100 arcsec. In otherexamples, the FWHM is less than 50 arcsec. In some examples, themicroscopic crystalline defects, and, specifically, the averagedislocation density of the GaN wafer is less than about 10⁷ cm⁻². Inother examples, the dislocation density is less than about 10⁶ cm⁻². Inother examples, the dislocation density is less than about 10⁵ cm⁻². Inother examples, the dislocation density is less than about 10⁴ cm⁻². Inother examples, the dislocation density is less than about 10³ cm⁻².

Certain implementations of the present invention may be furtherunderstood by following illustrative, non-limiting examples.

EXAMPLE 5 Semi-insulating GaN Boule Growth

In this example, we illustrate a semi-insulating GaN boule. Sapphire wasused as the starting substrate. Using the sputtering method disclosed inU.S. Pat. No. 6,784,085, an AlN layer approximately 0.7 μm thick wasgrown on the sapphire substrate for use as a nucleation layer for theHVPE GaN growth. X-ray diffraction was used to verify the AlN film wassingle crystal. The AlN/sapphire structure was loaded into thepreviously described vertical HVPE system 100 (FIG. 1) and the GaNgrowth was commenced.

FIG. 15 is an illustration of an example of a process 1500 for growingthe semi-insulating bulk GaN substrate or boule, including thetemperature 1504, NH₃ flows 1508, and HCl flows 1512 for the nucleationstage 1522, transition stage 1526, and bulk growth stage 1530. The bulkgrowth process 1500 included a nucleation step 1522 on the AlN-coatedsapphire substrate with a rough surface morphology, transitioning 1526the growth condition from the nucleation stage 1522 to the bulk growthstage 1530, and bulk growth 1530.

The nucleation layer consisted of an 18 μm thick layer with a pittedmorphology, grown at higher growth rate and lower growth temperaturethan the bulk layer. The growth rate for the nucleation layer was around225 microns per hour, the growth time was around 5 minutes, the growthtemperature was around 955° C., and the V:III ratio was around 15.

The transition to the bulk growth consisted of two layers. The firstlayer of about 7 μm thick was grown at a much reduced growth rate,resulting in a slightly less pitted morphology than the initialnucleation layer. The growth rate was around 50 microns per hour, thegrowth time was around 8 minutes, the growth temperature was around 955°C., and the V:III ratio was around 60. The second transition layerconsisted of a layer approximately 800 μm thick, which resulted in asurface nominally free of growth pits. The growth rate was around 100microns per hour, the growth time was around 8 hours, and the growthtemperature was around 970° C. During growth of the transition layer,the ammonia flow (and thus V:III ratio) was gradually reduced to achievethe desired conditions for bulk growth. The initial V:III ratio wasaround 28 and was reduced over the course of approximately four hours toa value around 16. Additionally, during growth of the transition layer,ferrocene was introduced into the growth reactor to incorporate iron asa compensating impurity and achieve semi-insulating behavior. The ironconcentration was approximately 1×10¹⁸ cm⁻³. The ferrocene flow was heldconstant during the remainder of the growth of both the transition layerand the bulk layer.

Following the transition layer, the bulk growth layer was grown. Thebulk layer was grown for at least 24 hours at the growth conditions atthe end of the transition layer. Depending on the final growth rate andtemperature, the NH₃ flow or the temperature or the growth rate or somecombination of the three were adjusted to prevent either pitting ormicrocracking from developing.

After completion of the growth sequence, the resulting GaN/sapphirebi-layer was cooled to room temperature at a cooling rate ofapproximately 6° C. per minute. During the cooling process, the sapphiresubstrate completely delaminated from the GaN/sapphire bi-layer forminga freestanding GaN substrate or boule. The GaN substrate in this examplewas 3 inches in diameter and the thickness of the substrate was about3.6 mm. FIG. 16 is an optical picture showing the top view of the GaNsubstrate obtained in this example. The surface of the boule still had afew pits. The GaN boule was sliced into 4 slices, and polished to amirror finish to remove the pits. The wafers were further subjected to achemical mechanical polishing step that eliminated surface andsubsurface damage.

The GaN crystal grown had very low crystal defects. The GaN boule wassliced into multiple wafers. FIG. 17 shows the dislocation density inthe iron-doped, semi-insulating GaN crystal as measured byroom-temperature cathodoluminescence (CL) imaging (panchromatic). In theCL imaging process, an electron beam is rastered across the sample andluminescence intensity is recorded. Crystal defects such as threadingdislocations act as non-radiative recombination centers and emit lessluminescence and thus exhibit dark spots in the CL image. Using thistechnique, the dislocation density was measured to be as low as about5×10⁴ cm⁻².

The high crystal quality of the GaN crystal grown in this example isalso confirmed with x-ray rocking curve measurement. FIG. 18 is thex-ray rocking curve for the (0002) reflection. The FWHM was about 90arcsec for the (0002) rocking curve, and was about 120 arcsec for theasymmetric (102) rocking curve.

The iron-doped GaN crystal is semi-insulating. A 10 mm×10 mm GaN piecewas obtained from the wafer or boule and the resistivity of the piecewas measured by Hall measurement and by contactless resistivity mapping(COREMA) techniques. FIG. 19 shows the resistivity and carrierconcentration as a function of temperature. The room temperatureresistivity of the sample was greater than about 2×10⁸ ohm-cm.

FIG. 20 is a COREMA resistivity map of a 10 mm×10 mm GaN substrateobtained from the iron-doped GaN boule. The mean room temperatureresistivity was about 1×10⁹ ohm-cm and the minimum resistivity measuredon the sample was about 5×10⁸ ohm-cm. Both the Hall and COREMAmeasurements demonstrated that the iron-doped GaN wafers weresemi-insulating.

EXAMPLE 6 Undoped GaN Boule Growth

In this example, we illustrate another embodiment of the presentinvention in which a high-purity undoped GaN substrate or boule wasgrown. Sapphire was used as the starting substrate. Using the sputteringmethod disclosed in U.S. Pat. No. 6,784,085, an AlN layer approximately0.7 μm thick was grown on the sapphire substrate for use as a nucleationlayer for the HVPE GaN growth. X-ray diffraction was used to verify theAlN film was single crystal. The AlN/sapphire structure was loaded intothe previously described vertical HVPE system 100 (FIG. 1) and the GaNgrowth was commenced.

FIG. 21 is an illustration of an example of a process 2100 for growingthe undoped bulk GaN substrate or boule with a pitted surfacemorphology, including the temperature 2104, NH₃ flows 2108, and HClflows 2112 for the nucleation stage 2122, transition stage 2126, andbulk growth stage 2130. The bulk growth process 2100 included anucleation step 2122 on the AlN-coated sapphire substrate with a roughsurface morphology, transitioning 2126 the growth condition from thenucleation stage 2122 to the bulk growth stage 2130, and bulk growth2130.

The nucleation layer consisted of a 10 μm thick layer with a pittedmorphology, grown at lower temperature and higher growth rate than thebulk growth step. The growth rate for the nucleation layer was around150 microns per hour, the growth time was around 5 minutes, the growthtemperature was around 950° C., and the V:III ratio was around 30. Atthe end of the growth of the nucleation layer, the surface issubstantially covered with small pits.

The transition to the bulk growth was accomplished by growing the GaNcrystal at a slower growth rate by initially reducing HCl flow andkeeping the same temperature and ammonia flow as for the nucleationlayer. The growth rate for the transitional layer was around 30 micronsper hour, the growth time was around 8 minutes, and the growthtemperature was around 950° C. Because the HCl flow was reduced and theammonia flow was kept the same, the V:III ratio was initially around150. During the growth of the second stage of the transition layer, theV:III ratio was gradually reduced to achieve the desired conditions forbulk growth. The growth rate was increased to around 100 microns perhour, the growth temperature was increased to around 955° C. and theV:III ratio was reduced to around 30. The density of pits on the growingGaN surface was reduced and the size of the pits was increased duringthe growth of the transitional layer.

Following growth of the transition layer, the bulk layer was grown. Thebulk layer was grown for at least 16 hours at the growth conditions atthe end of growth of the transition layer. Depending on the final growthrate and temperature, the NH₃ flow or the temperature or the growth rateor some combination of the three were adjusted to maintain the coverageof the surface pits and faceted growth morphology.

After the completion of the growth, the resulting GaN/sapphire bi-layerwas cooled to room temperature. During the cooling down process, thesapphire substrate completely delaminated from the GaN/sapphirebi-layer, forming a freestanding crack-free GaN article (substrate,boule, etc.). FIG. 22 is an optical picture of the GaN article obtainedin this example. The GaN article was 2 inches in diameter and thethickness of the article was about 1.6 mm. The surface of article hadsome pits that were removed during the wafer polishing process.

The freestanding GaN article was sliced into two wafers and wafers weremechanically lapped and polished in several stages with progressivelysmaller diamond slurries to achieve a mirror finish on the front side.The backside is also lapped to achieve a matte finish. Optionally, thebackside can also be polished to a mirror finish. The lap/polish processof both front and back sides of the substrate ensures the final waferhas minimal thickness variation across the wafer and minimal bow. Achemical mechanical polish (CMP) process was used as the final polishstep, which produced a damage-free surface. Other suitable surfacefinishing techniques may alternately or additionally be employed.

The dislocation density of the wafer was measured using a CL imagingtechnique. FIG. 23 is a CL image of an area of the wafer produced inthis example, showing the dislocation density of the wafer was about8×10⁶ cm⁻².

EXAMPLE 7 Si-doped Conductive GaN Boule with Smooth Surface Morphology

This example illustrates an embodiment of the present invention in whichthe GaN was doped with silicon for n-type conductivity and the surfaceof the boule during the growth was maintained essentially pit-free.Sapphire was used as the starting substrate. Using the sputtering methoddisclosed in U.S. Pat. No. 6,784,085, an AlN layer approximately 0.7 μmthick was grown on the sapphire substrate for use as a nucleation layerfor the HVPE GaN growth. X-ray diffraction was used to verify the AlNfilm was single crystal. The AlN/sapphire structure was loaded into thepreviously described vertical HVPE system and the GaN growth wascommenced.

FIG. 24 is an illustration of an example of a growth process 2400 forn-type, bulk GaN with a smooth surface morphology, including thetemperature 2404, NH₃ flows 2408, and HCl flows 2412 for the nucleationstage 2422, transition stage 2426, and bulk growth stage 2430. The bulkgrowth process 2400 included a nucleation step 2422 on the AlN-coatedsapphire substrate with a rough surface morphology, transitioning 2426the growth condition from the nucleation stage 2422 to the bulk growthstage 2430, and bulk growth 2430. Silane with a specific flow rate andconcentration to achieve [Si]=1−5×10¹⁸ cm⁻³ was delivered into thegrowth system during the nucleation 2422, transition 2426, and bulkgrowth 2430. In this example, the temperature of the growth was keptconstant while the growth rate and V:III ratio was varied to achievedesirable nucleation, transition and bulk growth.

The nucleation layer consisted of a 14˜20 μm thick layer with a pittedmorphology that was achieved with a higher V:III ratio than the bulkgrowth. The growth rate of the nucleation rate was about 300 microns perhour. The growth time was around 5 minutes, the growth temperature wasaround 1000° C., and the V:III ratio was around 75. At the end of growthof the nucleation layer, the surface of the GaN film was covered withpits.

The transition to the bulk growth used a lower V:III ratio of 16 toachieve a smooth surface morphology for bulk growth. The growth rate wasabout 300 μm per hour. The growth time was around 2 hours, and thegrowth temperature was around 1000° C. During growth of the transitionlayer, the V:III ratio was gradually reduced in 5 minutes and then keptconstant. This transition layer thickness was approximately 600 μm,which resulted in a surface nominally free of growth pits.

Following growth of the transition layer, the bulk growth layer wasgrown and a higher V:III ratio of 25 was used to maintain a nominallypit free surface and to prevent any possible defects, such asmicrocracking, from occurring during bulk stage. The total growth timewas 40 hours and the length of the boule was about 1.2 centimeters.

EXAMPLE 8 Si-doped GaN Boule with Faceted Surface Morphology

In this example, we illustrate an embodiment of the invention where theGaN boule was grown with a faceted surface morphology during the boulegrowth. Sapphire was used as the starting substrate. Using thesputtering method disclosed in U.S. Pat. No. 6,784,085, an AlN layerapproximately 0.7 μm thick was grown on the sapphire substrate for useas a nucleation layer for the HVPE GaN growth. X-ray diffraction wasused to verify the AlN film was single crystal. The AlN/sapphirestructure was loaded into the previously described vertical HVPE systemand the GaN growth was commenced.

FIG. 25 is an illustration of an example of a growth process 2500 forn-type, bulk GaN with a pitted surface morphology, including thetemperature 2504, NH₃ flows 2508, and HCl flows 2512 for the nucleationstage 2522, transition stages 2526 and 2528, and bulk growth stage 2530.The bulk growth process 2500 included a nucleation step 2522 on theAlN-coated sapphire substrate with a rough surface morphology,transitioning 2526 the growth condition from the nucleation stage to alow pitting growth stage, a second transition stage 2528 where acontrolled level of pitting and surface facet was established, and bulkgrowth 2530. Silane with a specific flow rate and concentration toachieve a [Si]=1−5×10¹⁸ cm⁻³ was delivered into the growth system duringthe nucleation 2522, transition 2526 and 2528, and bulk growth 2530 forn-type doping. The growth temperature was held constant whereas thegrowth rate and V:III ratio was adjusted in the various stages of thegrowth.

The nucleation layer consisted of a 14˜20 μm thick layer with a pittedmorphology grown at a higher V:III ratio. The growth rate of thenucleation layer was about 300 microns per hour to achieve desiredpitting level. The growth time was around 5 minutes, the growthtemperature was around 1000° C., and the V:III ratio was around 75.

The first transition layer used a lower V:III ratio such as 16 toachieve a smooth morphology for bulk growth. This transition layer wasapproximately 600 μm thick. The growth time was around 2 hours, and thegrowth temperature was around 1000° C. During growth of the transitionlayer, the V:III ratio was gradually reduced to achieve the desiredconditions in 5 minutes. It was important to achieve a smooth surfacemorphology and eliminate any large pits in the first transition layerprior to controllably introducing a pitted morphology, in order toprevent any pits from exceeding the desired size during the bulk growth.After growth of the first transition layer, a second transition layerwas grown which resulted in a surface with a nominally uniformdistribution of growth pits between 50 and 500 microns in size. Thegrowth rate was around 300 microns per hour to achieve desired pittinglevel. The growth time was around 1 hour, the growth temperature wasaround 1000° C., and the V:III ratio was around 50.

Following growth of the second transition layer, the bulk growth layerwas grown under conditions to maintain the pit size and distribution,and to prevent any possible defects, such as microcracking orpolycrystalline GaN inclusions, from occurring during bulk stage. TheNH₃ flow was lowered to prevent excessive growth in the pit size and toprevent polycrystalline GaN inclusions from forming. Depending on thefinal growth rate and temperature, the NH₃ flow or the temperature orthe growth rate or some combination of the three were adjusted tomaintain the surface morphology. The growth condition for this examplewas: growth rate about 300 microns per hour, V:III ratio 35, growthtemperature 1000° C., and growth time 40 hours. A boule of about 1.2centimeter in length was obtained.

The GaN boule was sliced into 17 wafers using a multiwire wire saw. Thewafers were lapped and polished to mirror finish. A chemical mechanicalpolish was used as the final polishing step that removed surface andsubsurface damage.

Freestanding Group III Nitride Crystals and Methods for Producing Sameby Self-Separation

Thermal Stress and Cracking of GaN Grown on Sapphire

After discovering that HVPE GaN grown under conditions that yield pittedsurface morphology eliminates the microcracks in GaN during the growth,we encountered another problem: cracking of GaN and sapphire when thewafer cooled down from the growth temperature to the ambient roomtemperature due to the mismatch in the coefficients of thermalexpansion.

A sapphire substrate has a larger coefficient of thermal expansion (CTE)than a gallium nitride thin or thick film. During cooling down from agrowth temperature of, for example, about 1000° C. to room temperature,sapphire shrinks faster than the gallium nitride film. In a simpletwo-layer model, sapphire near the interface is under biaxial tensilestress and the GaN film near the interface is under biaxial compressivestress in the plane parallel to the interface. It is noted that thegallium nitride at the interface is always under biaxial compressivestress, whereas the stress on the gallium nitride surface depends on thethickness of the gallium nitride film (for a given sapphire substratethickness). The stress on the gallium nitride surface is changed fromcompressive stress to tensile stress with increased gallium nitride filmthickness. The calculated thermal stresses in GaN and sapphire for asimple two-layer model as a function of GaN layer thickness is shown inFIG. 26.

The thermal stress results in high strain energy in the GaN/sapphirecomposite. The strain energy may be released by cracking. Crackingoccurs, in general, if the strain is above the critical strain andenergy is released by cracking. For example, when the GaN film is quitethin and the sapphire is thick, the GaN/sapphire composite does notcrack after cooling because the GaN is under compressive stress andsapphire is under tensile stress, which is still below the criticalstress. When both the GaN layer and sapphire substrate are relativelythick, sapphire cracks under the tensile stress, which can propagateinto the GaN film, causing GaN cracking as well.

There are four main possible types of crackings when considering theGaN/sapphire bilayer structure: cracks perpendicular to the surface andcracks parallel to the surface; and cracks in the GaN and sapphire.Cracks perpendicular to the surface break the wafer. Cracks in the GaNlayer parallel to the surface (lateral cracks) can separate GaN from thesapphire substrate. Sometimes both types of cracks are present, whichcan result in small pieces of freestanding GaN.

Methods for Producing GaN Wafer by Self-Separation

An example of a GaN growth method that affects the stresses of the GaNfilm on a substrate such as sapphire will now be described. In general,a hetero-structure consisting substantially of a GaN/substrate bi-layeror composite is created such that, upon cool down from the growthtemperature to room temperature, cracking perpendicular to the surfaceof a layer is greatly suppressed and lateral cracks in the GaN layerparallel to the surface are promoted to cause separation between themajority of the GaN layer and the underlying substrate. After theseparation, a stress-free GaN wafer as large as the substrate, and thesubstrate with a thin GaN film, are obtained. This method of crackingmay be strongly dependent on the substrate utilized, the nucleationlayer utilized, the initial growth conditions and the bulk growthconditions of the GaN layers.

The GaN growth method in some implementations may include several growthsteps, including depositing an epitaxial nitride (e.g., AlN) layer,growing a thin GaN layer under a 3D growth mode with a surface coveredwith pits, growing a recovery GaN layer to recover from a much pittedsurface morphology to a less pitted surface morphology, and growing abulk growth layer.

The GaN growth method according to this example will now be describedwith reference to FIG. 27, which schematically illustrates a GaN crystalstructure 2700, and FIG. 28, which schematically illustrates a GaNcrystal/substrate bi-layer 2800 after self-separation.

Referring to FIG. 27, a suitable substrate 2704 is provided. In someimplementations, the substrate 2704 may have a characteristic dimension(e.g., diameter) of about 2 inches or greater. As further examples, thediameter of the substrate 2704 may be about 3″ or greater, about 4″ orgreater, or any other suitable size such as about 12″ or greater. Thesubstrate 2704 may be sapphire (Al₂O₃), although other suitablesingle-crystal substrates 2704 may be utilized.

The first step of the growth process is to deposit a thin epitaxialnitride layer 2708 on the substrate 2704. The purpose of this epitaxialnitride layer 2708 is to provide a template for epitaxial growth of GaN.The epitaxial nitride layer 2708 in one embodiment is prepared byhigh-temperature sputtering in a sputtering chamber. The epitaxialnitride layer 2708 may also be formed by molecular beam epitaxy (MBE),metal-organic vapor phase epitaxy (MOVPE or MOCVD), hydride vapor phaseepitaxy, or high-temperature annealing of the substrate 2704 in ammonia.In one example, the thickness of the epitaxial nitride layer 2708 is inthe range (ranges) from about 0.05 to about 10 microns. In anotherexample, the thickness of the epitaxial nitride layer 2708 is typicallyabout 0.2 to about 2 microns. Other types of template layers may beused, for example, a GaN or AlGaN layer, grown by MOVPE, MBE or HVPE.

The second step of the growth process is to grow a single-crystallineGaN layer 2712 by hydride vapor phase expitaxy in a 3D growth mode. Thenitride-coated substrate 2704/2708 is loaded into an HVPE reactor, andthe reactor is purged with high-purity nitrogen to remove impurities. Alayer 2712 of gallium nitride is then grown on the epitaxial nitridelayer 2708. This GaN layer 2712 is grown in a three-dimensional (3D)growth mode, where the surface of the film is very rough and coveredwith pits.

The growth conditions for the 3D growth mode are the conditions thatpreferentially produce a pitted surface morphology. Such conditions are,for example, higher growth rate, and/or higher ammonia flow (or V:IIIratio), and/or lower growth temperature than the “optimal” thin-filmgrowth condition. The “optimal” thin-film growth condition is one thatwould produce smooth, substantially pit-free, crack-free thin films(e.g., with a thickness equal to or less than 3 microns), but wouldproduce microcracked thick films (e.g., with a thickness equal to orgreater than 20 microns). Example 1, given above, is an example of anoptimized growth condition for a thin film. When growing a thin film (≦3μm), this “optimal” thin-film growth condition typically produces acrack-free film, whereas when growing a thick film (≧20 μm), this“optimal” growth condition typically produces a microcracked film.

In growing the 3D-growth layer 2712, for a given growth temperature,there is a preferred growth rate and V:III ratio to achieve the desiredresult. As an example, for a growth temperature of about 940° C., thegrowth rate for 3D growth mode is approximately 190-200 μm/hr at a V:IIIratio of around 17-18. A thickness of 15-20 μm has been shown to resultin the desired outcome. As another example, for a growth temperature ofabout 980° C., the growth rate for 3D mode growth is approximately 250μm/hr at a V:III ratio of about 50, and a thickness of 25-30 μm hasshown similar results. In other examples, other growth conditions canachieve similar results.

There are two purposes for this 3D growth layer 2712: first is toprevent microcracking of GaN during the growth, and second is to presentthe GaN film with a certain stress condition that will facilitate thelateral cracks during cool down. In one example, the thickness of the 3Dgrowth layer 2712 may range from about 5 to about 500 microns. Inanother example, the thickness of the 3D growth layer 2712 may rangefrom about 5 to about 200 microns. In another example, the thickness ofthe 3D growth layer 2712 may range from about 5 to about 100 microns. Inanother example, the thickness of the growth layer 2712 ranges fromabout 10 to about 50 microns. In yet another example, the thickness ofthe 3D growth layer 2712 ranges from about 20 microns to about 30microns. In another example, the thickness of the 3D growth layer 2712is about 20 microns.

At the end of 3D nucleation layer growth, the growth surface hassubstantial amounts of pits. The pitting percentage of the 3D nucleationlayer, defined as the percentage of the surface covered with pits, maybe greater than about 50%. In other examples, the pitting percentage isgreater than about 75%. In other examples, the pitting percentage isgreater than about 90%.

In one implementation, the growth temperature in the 3D growth moderanges from about 900° C. to about 1000° C., the V:III ratio in the 3Dgrowth mode ranges from about 10 to about 100, and the growth rate inthe 3D growth mode ranges from about 50 μm/hr to about 500 μm/hr.

If the GaN film is grown under the 3D growth mode with higher thickness,the GaN film quality is gradually changed from an epitaxialsingle-crystalline film to a polycrystalline film.

The third step of the growth process is to change the growth conditionsto recover the surface morphology from a highly pitted surfacemorphology to a less pitted surface morphology, which prepares thesurface of the growing crystal for the subsequent bulk growth stagewhere most of the GaN crystal is grown. In this third step, the growthmode is gradually changed from a 3D growth mode to a 2D growth mode.This transition is accomplished by growing a recovery layer 2716 on the3D growth layer 2712 under conditions such as lower growth rate, and/orlower ammonia partial pressure, and/or higher growth temperature thanthe growth condition of the 3D nucleation layer 2712. In one example,the thickness of the morphology recovery layer 2716 may range from about5 to about 500 microns. In another example, the thickness of themorphology recovery layer 2716 may range from about 5 to about 200microns. In another example, the thickness of the morphology recoverylayer 2716 may range from about 5 to about 100 microns. In anotherexample, the thickness of the recovery layer 2716 ranges from about 5 toabout 50 microns. In another example, the thickness of the recoverylayer 2716 is about 8 microns. The purposes of the recovery layer 2716are to prevent the GaN film from turning into polycrystalline, and toobtain a film stress state that facilitates lateral cracks during cooldown.

In one implementation, the growth temperature in the recovery layergrowth mode ranges from about 920° C. to about 1100° C., the V:III ratioin the recovery layer growth mode ranges from about 8 to about 80, andthe growth rate in the recovery layer growth mode ranges from about 50μm/hr to about 500 μm/hr.

The fourth growth step is the bulk growth step where the bulk of the GaNfilm is grown. As illustrated in FIG. 27, a bulk layer or crystal 2720is grown on the recovery layer 2716. The growth condition is chosen sothat the morphology of the GaN film is slightly pitted or pit-free. TheGaN growth mode in this step is substantially a 2D growth mode. Thegrowth conditions may be held constant during this step. Alternatively,the growth condition may be slightly ramped, for example, slightlyramping down ammonia flow or slightly ramping down the growth rate orslightly ramping up the temperature. The purpose of the ramping in thebulk growth step is to further reduce the density of the pits on thegrowing GaN surface. During the bulk growth step, the density of thepits on the growing GaN surface is gradually reduced. At the end of thebulk growth, the GaN surface is slightly pitted or pit-free. In oneexample, the thickness of the GaN bulk layer 2720 grown in the bulkgrowth step ranges from about 500 to about 2000 microns (0.5 to 2 mm).In another example, the thickness of the GaN bulk layer 2720 ranges fromabout 1000 to about 1500 microns (1 to 1.5 mm). In some implementations,the crystal growth process is performed to yield a single wafer. Inother implementations, the process may be performed to grow a GaN boulethat can be sliced into multiple wafers, in which case a thicker bulklayer 2720 may be grown, for example, about 2 mm or greater, from about2 mm to about 10 mm, or about 10 mm or greater.

In one implementation, the growth temperature in the bulk growth stageranges from about 950° C. to about 1100° C., the V:III ratio in the bulkgrowth stage ranges from about 5 to about 50, and the growth rate in thebulk growth stage ranges from about 50 μm/hr to about 500 μm/hr.

After completing the growth, the resulting thick GaN-on-substratebi-layer is gradually cooled down. In one example, the cooling rate isless than about 20° C. per minute, and in another example is less thanabout 10° C. per minute. In another example, the rate of cooling isabout 6° C. per minute. During this cool down time, lateral crackingoccurs in the GaN film with the crack plane substantially or essentiallyparallel to the GaN/substrate interface, leading to the separation ofGaN from the underlying substrate.

FIG. 28 illustrates the resulting separated GaN/substrate bi-layerstructure 2800. A thick GaN wafer 2822 having a characteristic dimension(e.g., diameter) as large as the initial substrate 2704 (FIG. 27) may beobtained, along with the substrate 2806 covered with a thin layer ofGaN. As examples, when a 2″ substrate 2704 is utilized, a 2″ GaN wafer2822 may be obtained. When a 3″ substrate 2704 is utilized, a 3″ GaNwafer 2822 may be obtained. The substrate 2806 may remain intact, orremain partially intact with edge fracture, or fracture into severalpieces. The remaining GaN on the substrate 2806 is typically less than500 microns thick. The thickness of the freestanding GaN wafer 2822typically ranges from about 0.5 mm to about 10 mm.

When implementing methods of the invention, the lateral cracks occur inthe GaN/substrate bi-layer structure 2800 because it is the mosteffective way to relieve the thermal stress. We wish not to be bound byany particular theory regarding how the lateral cracks occur, but herewe present a possible mechanism by which lateral cracking in thebi-layer structure 2800 may occur. Since a substrate material such assapphire shrinks more than GaN during cool down, the thermal stresscondition of the GaN/substrate bi-layer structure 2800 results in thesubstrate being under tensile stress whereas the GaN near the interfaceis under the compressive stress. Film fracture behavior undercompressive stress has been reported in the open literature (see, forexample, “Fracture in Thin Films,” S. Zuo, Encyclopedia of Materials:Science and Technology, Elsevier Science, 2001) and may be used todescribe the GaN/substrate system. In accordance with methods describedin the present disclosure, when a film is under compression, it canself-separate, or debond, from the underlying substrate.

In general, fracture is driven by the relaxation of residual stresses,in this case, thermal stress due to expansion mismatch. Fracture willoccur if the driving force exceeds the fracture resistance for theparticular fracture mechanism. Cracks can form at pre-existing flaws inthe film, in the substrate, or at areas where the resolved stress isconcentrated and exceeds the critical fracture value. The mechanics andprocess of the fracture processes are not fully understood, but underbiaxial compressive stress, the in-plane lattice constants a and b ofthe GaN film are shortened whereas the lattice constant c is elongated.The increased lattice constant c under the biaxial stress weakens thebond strength causing the separation in the c direction, i.e., lateralcracking. This stress may be increased locally by geometric factors.

In the present case, the structure of the as-grown GaN layer(s) leads toconditions conducive to compressive debonding at or near the interfacebetween the GaN and substrate. The epitaxial nitride (e.g., AlN) layer,the first GaN nucleation step, the second GaN nucleation step, or somecombination of the three, results in the initiation of the debondingbehavior during cool down. Experimentally, the initial debonding hasbeen observed to occur near the center of the substrate bi-layer. Afterdebonding, buckling occurs as the self-separated area increases. As theelastic constants of the GaN and the substrate (e.g., sapphire) areclose in magnitude and plastic deformation is extremely small, thedegree of buckling is limited. After buckling, both normal and shearstresses develop that will continue to grow the debonded area like acrack. Under preferred conditions, the debonding continues to the edgeof the GaN/substrate bi-layer and separates the GaN layer from thesubstrate.

There are three major competing processes that have also been observedfor the relief of the thermal stress in the GaN/substrate bi-layer: (1)lateral cracking in the GaN layer due to compressive stress in the GaNlayer near the GaN/substrate interface, to form a whole piece of thickstress-free GaN wafer and a thinner-GaN/substrate bi-layer, (2) verticalcracks in sapphire due to tensile stress developed during cool down (thevertical cracks may also propagate to GaN), and (3) the presence of bothlateral cracking in GaN and vertical cracking in substrate and GaN. Acrack will occur in the direction where the stress exceeds the criticalstress.

In addition to the thermal stress during cool down, the GaN/substratebi-layer also experiences a growth stress built up during the crystalgrowth. The growth stress can lead to GaN microcracks under certaingrowth conditions as discussed in the previous paragraphs. The growthstress can also lead to the breaking of the substrate and the GaN layerduring the growth, which is observed under non-optimal growthconditions.

The growth sequence employed in the present invention prepares in theGaN layer a certain stress state during the growth, such as increasedcompressive stress in the GaN layer near the interface or reducedtensile stress on the substrate, enabling the compressive stressdebonding during cool down while preventing growth stress leading tomicrocracking or vertical cracking in the substrate or GaN layer,yielding a freestanding GaN substrate. The formation of the 3Dnucleation layer is one key aspect for the separation of the bulk GaNlayer during cool down. If the first nucleation layer is grown underconditions of 2D growth, the most likely mechanism for thermal stressrelief for the thick GaN/substrate bi-layer is vertical cracking in thesubstrate that propagates to the GaN layer.

In some implementations, during the bulk growth, no impurity isintroduced and all the gas sources are purified in order to achievehigh-purity GaN crystals. In some examples, the impurity concentrationin the high-purity GaN layer is less than about 10¹⁷ cm⁻³. In otherexamples, the impurity concentration is less than about 10¹⁶ cm⁻³. Inother examples, the impurity concentration is less than about 10¹⁵ cm⁻³.

Alternatively, in other implementations, during the bulk growth, n-typeimpurities, such as silicon (introduced, for example, as diluted silane)or oxygen, are introduced to produce n-type GaN crystals. Theintroduction of impurities may occur at any stage of the GaN growth. Insome examples, the electron concentration in the n-type bulk GaN layeris greater than about 10¹⁷ cm⁻³. In other examples, the electronconcentration is greater than about 10¹⁸ cm⁻³. In other examples, theelectron concentration is greater than about 10¹⁹ cm⁻³. In someexamples, the resistivity of the n-type GaN layer is less than about 0.1ohm-cm. In other examples, the resistivity is less than about 0.01ohm-cm. In other examples, the resistivity is less than about 0.001ohm-cm.

In other implementations, during the bulk growth, p-type impurities suchas magnesium (Mg) are introduced to produce p-type GaN crystals. Mg maybe introduced as metal-organic compound or as metal vapor. Theintroduction of impurities may occur at any stage of the growth. In someexamples, the hole concentration in the p-type bulk GaN layer is greaterthan about 10¹⁷ cm⁻³. In other examples, the hole concentration isgreater than about 10¹⁸ cm⁻³. In other examples, the hole concentrationis greater than about 10¹⁹ cm⁻³. In some examples, the resistivity ofthe p-type GaN layer is less than about 0.1 ohm-cm. In other examples,the resistivity is less than about 0.01 ohm-cm. In other examples, theresistivity is less than about 0.001 ohm-cm.

In other implementations, the electrical properties of the bulk GaNcrystal can also be made semi-insulating by introducing a deep-levelacceptor. Transition metals, such as iron, are deep level acceptors.Iron may be introduced to the deposition zone either via a metal-organiccompound such as ferrocene or via iron chloride formed by reactingmetallic iron with hydrochloric acid. The concentration of thetransition metal in the bulk GaN layer may range from about 10¹⁶ toabout 10²⁰ cm⁻³. In other examples, the concentration of the transitionmetal ranges from about 10¹⁷ to about 10¹⁹. In other examples, theconcentration of the transition metal is around 10¹⁸ cm⁻³. Theresistivity of the SI bulk GaN layer at room temperature may be greaterthan about 10⁶ ohm-cm. In other examples, the resistivity is greaterthan about 10⁷ ohm-cm. In other examples, the resistivity is greaterthan about 10⁸ ohm-cm.

After self-separation, the GaN wafers may be mechanically polished to aspecified wafer thickness. To remove the subsurface damage, the wafermay be chemically mechanically polished as the last step. Reactive ionetching or inductively coupled plasma etching may also be used to removethe damaged surface layer. Other suitable surface finishing techniquesmay alternately or additionally be employed.

Example 5, described above in conjunction with FIGS. 15-20, may bereferred to as an example of a process for fabricating a free-standing,semi-insulating GaN substrate by self-separation. That example utilizesa growth sequence and growth conditions that produce a GaN/sapphirebi-layer. The GaN/sapphire bi-layer may be cooled to room temperature ata prescribed cooling rate that results in the sapphire substratecompletely delaminating from the GaN/sapphire bi-layer, thereby formingthe freestanding GaN substrate.

Example 6, described above in conjunction with FIGS. 21-23, may bereferred to as an example of a process for fabricating a free-standing,undoped GaN substrate by self-separation. That example likewise utilizesa growth sequence and growth conditions that produce a GaN/sapphirebi-layer. The GaN/sapphire bi-layer may be cooled to room temperature ata prescribed cooling rate that results in the sapphire substratecompletely delaminating from the GaN/sapphire bi-layer, thereby formingthe freestanding GaN substrate.

The techniques described in the present disclosure for makingfree-standing GaN substrates differ from those of the prior art. In oneaspect, methods of the present disclosure generate lateral cracks in GaNto separate GaN from the substrate. In another aspect, methods of thepresent disclosure employ several layers of GaN grown under differentconditions to control the stress in the GaN film and promote theseparation of GaN from the underlying substrate. In another aspect,methods of the present disclosure do not rely on the creation of voidsin the films.

Low Defect Group III Nitride Films Useful for Electronic andOptoelectric Devices and Methods for Making the Same

The pitted surface morphology referred to in the above discussionseliminates the microcracks in the GaN film during the HVPE GaN growth.However, the surface is not desirable as a foundation for further growthof some GaN-based device structures. The present invention disclosesmethods for growing high-quality, low defect density, pit-free andcrack-free GaN films by hydride vapor phase epitaxy. The GaN films aresuitable for the further growth of electronic and optoelectronic devicesbased on group III nitride alloys.

The GaN growth method in this implementation may include several growthsteps, including depositing an epitaxial nitride template layer on asuitable substrate, growing a thin GaN layer on the nitride-coatedsubstrate under a condition that yields a surface covered with pits, andgrowing a GaN layer on or from the pitted GaN layer under a conditionthat fills the pits and yields a pit-free surface.

According to this implementation, the first step of the growth processis to deposit a thin epitaxial nitride (e.g., AlN) layer on a suitablesubstrate such as, for example, sapphire. The purpose of this epitaxialnitride layer is to provide a template for epitaxial growth of GaN.Without the epitaxial nitride template, the HVPE GaN film grown on asubstrate such as sapphire under typical conditions may bepolycrystalline. The epitaxial nitride layer in one implementation isprepared by high-temperature reactive sputtering in a sputteringchamber. For example, an aluminum target and an AC plasma of an inertgas or gas mixture (e.g., an Ar/N₂ gas mixture) may be utilized todeposit the epitaxial nitride layer on a heated substrate. The epitaxialnitride layer may alternatively be formed by molecular beam epitaxy(MBE), metal-organic vapor phase epitaxy (MOVPE or MOCVD), hydride vaporphase epitaxy, or high-temperature annealing in ammonia. In one example,the thickness of the epitaxial nitride layer is in the range (ranges)from about 0.05 to about 2 microns. In another example, the thickness ofthe epitaxial nitride layer ranges from about 0.2 to about 2 microns.Other types of template layers may alternatively be used, for example,GaN or AlGaN layers, grown by MOVPE, MBE or HVPE.

The second step of the growth process is to grow a GaN layer by hydridevapor phase epitaxy in a growth condition that yields a pitted surfacemorphology. The nitride-coated substrate is loaded into a HVPE reactor,and the reactor may be purged with high purity nitrogen to removeimpurities. A layer of gallium nitride is then grown on the epitaxialnitride layer. The growth condition for this GaN layer is typicallyhigher growth rate, and/or higher ammonia flow (or V:III ratio), and/orlower growth temperature than the “optimal” thin-film growth condition.The “optimal” thin-film growth condition is one that would producesmooth, substantially pit-free, crack-free thin films (e.g., with athickness equal to or less than 3 microns), but would producemicrocracked thick films (e.g., with a thickness equal to or greaterthan 20 microns). Example 1, given above, is an example of an optimizedgrowth condition for a thin film. When growing a thin film (≦3 μm), this“optimal” thin-film growth condition typically produces a crack-freefilm, whereas when growing a thick film (≧20 μm), the “optimal” growthconditions typically produces a microcracked film.

The GaN film grown under the growth condition of this second step isvery rough and covered with pits. There are two purposes for this pittedlayer: first is to prevent microcracking of GaN during the growth; andsecond is to promote annihilation of dislocations. In one example, thethickness of this pitted layer ranges from approximately 2 toapproximately 50 microns. In another example, the thickness of thispitted layer ranges from approximately 5 to approximately 50 microns. Ifthe GaN film is grown under the pitted growth condition with higherthickness, the GaN film quality is gradually changed from an epitaxialsingle-crystalline film to a polycrystalline film.

In one implementation, the growth temperature during growth of the first(pitted) epitaxial GaN layer ranges from about 900° C. to about 1000°C., the V:III ratio ranges from about 10 to about 100, and the growthrate ranges from about 50 μm/hr to about 500 μm/hr.

The third step of the growth process is to grow an additional GaN layerunder conditions that cause the pits to be filled and yield a pit-freeand crack-free surface. The growth condition for this layer is typicallylower growth rate, and/or lower ammonia partial pressure, and/or highergrowth temperature than the growth condition utilized for the pittedlayer. The thickness of this layer is in one example greater than about3 microns, in another example greater than about 5 microns, and inanother example greater than about 10 microns. In another example, thethickness of second epitaxial GaN layer ranges from about 3 to about 200microns. In another example, the thickness of second epitaxial GaN layerranges from about 8 to about 200 microns. The optimal thickness of thepit-free layer depends on the thickness of the pitted layer. A thickerlayer grown under pitted growth conditions correspondingly requires athicker layer grown under pit-free conditions to completely fill thepits. The ratio of the thickness of the layer grown under pittedconditions to the thickness of the layer grown under pit-free conditionis in one example between about 2:1 and about 1:5.

In one implementation, the growth temperature during growth of thesecond epitaxial GaN layer ranges from about 920° C. to about 1100° C.,the V:III ratio ranges from about 8 to about 80, and the growth rateranges from about 5 μm/hr to about 500 μm/hr.

FIG. 29 is a schematic illustration of an exemplary growth process 2900of the present invention. First, a substrate 2904 is provided. Anepitaxial nitride (e.g., AlN) layer 2908 is then deposited on thesubstrate 2904. The deposition of the epitaxial nitride layer 2908 maybe done in the same reactor for the subsequent GaN growth or in adifferent deposition chamber. GaN material is subsequently deposited onthe nitride-coated substrate 2904/2908 by hydride vapor phase epitaxy intwo steps with different growth conditions. A first GaN layer 2912 isgrown under a condition that results in a pitted surface morphology, andsuch conditions are characterized by relatively higher growth rate,and/or high ammonia flow, and/or lower growth temperature than utilizedduring the second GaN growth step. A second GaN layer 2916 is then grownunder a condition that fills the pits on the surface 2914 of the firstGaN layer 2912 and yields pit-free smooth GaN layer, and such growthconditions are characterized by relatively lower growth rate, and/orlower ammonia flow, and/or higher growth temperature than employed inthe first, pitted-growth step. The combination of the two GaN growthsteps both eliminates the GaN microcracking during the growth andprovides a smooth, low-defect GaN surface 2918 that is suitable for thefurther growth of devices based on Group III nitrides. The growthprocess 2900 yields a GaN film generally depicted at 2924 in FIG. 29.

The substrate 2904 may be any substrate that has a surface having a3-fold symmetry or close to having a 3-fold symmetry. Some examples ofthe present disclosure utilize c-plane sapphire as the substrate 2904.Other substrates 2904 such as silicon, silicon carbide, diamond, lithiumgallate, lithium aluminate, zinc oxide, spinel, magnesium oxide, andgallium arsenide may be utilized for the growth of low-defect,crack-free GaN films. In one example, the substrate 2904 has acharacteristic dimension (e.g., diameter) of about 2 inches or greater.In other examples, the diameter of the substrate 2904 is about 2″ orgreater, about 3″ or greater, about 4″ or greater, or any other suitablesize.

The substrate surface 2906 may be exactly c-plane or vicinal surfaces ofthe c-plane. Vicinal surfaces may promote step-flow during the HVPE GaNgrowth and may yield smoother surface morphology. The offcut angle ofthe vicinal surface with respect to the c-plane in one example rangesfrom about 0° to about 10°, in another example from about 0.1° to about10°, and in another example from about 0.5° to about 5°. The directionof offcut may be along the <1-100> direction or along the <11-20>direction, or along a direction between <1-100> and <11-20>.

In some implementations, the deposition of the epitaxial nitride layer2908 may be needed to grow single-crystalline GaN films on substrates2904 such as sapphire substrates using the HVPE process. In oneimplementation, the epitaxial nitride layer 2908 is deposited byreactive sputtering on a heated substrate 2904 in a sputter depositionchamber. The nitride-coated substrate 2904/2908 is subsequently removedfrom the sputter chamber and loaded into the HVPE reactor for GaNgrowth. As alternatives to depositing AlN by HVPE, other nitride layers,such as AlN grown by MOCVD, GaN grown by MOCVD, AlGaN grown by MOCVD,and the like may also be used. A reactive sputtering-deposited AlN layerhas the advantage of lower cost than MOCVD or MBE deposited nitridelayers. AlN layers may also be grown in the HVPE reactor byincorporating an Al source so that hydrochloric acid reacts with Al toform aluminum chloride that reacts with ammonia in the deposition zoneto form AlN on the substrate surface 2906.

The growth of GaN film 2924 according to this implementation includes atleast two growth steps with different growth conditions. The growthtemperature is typically between 900° C. and 1100° C., the growth rateis typically between 5 and 500 microns per hour, and V:III ratio istypically between 5 and 100. The two-step GaN growth is characterized bythe growth conditions of the first step having lower growth temperature,and/or higher ammonia flow, and/or higher growth rate than the secondstep. In one example, the growth temperature is about 15° C. hotter inthe second step than in the first step, and the growth rate of thesecond step is about one-fourth of the first step. At the end of thefirst step, the GaN surface 2914 is rough and covered with the pits. Ifthe growth is stopped at the end of the first step and wafer is takenout of the reactor, the GaN surface 2914 is not specular, as shown in amicrophotograph in FIG. 30. The pit coverage, defined as the percentageof a surface covered with the pits on the surface, is in one examplegreater than about 50%, and in another example greater than about 75%,and in another example greater than about 90% at the end of the firstGaN growth step. At the end of the second step, the GaN surface 2918 issmooth, specular and pit-free. The pit coverage in the final film is inone example less than 1%, in another example less than 0.1%, and inanother example less than 0.01%.

The resulting GaN film 2924 may have a characteristic dimension (e.g.,diameter) as large as the initial substrate 2904. As examples, when a 2″substrate 2904 is utilized, a 2″ GaN film 2924 may be obtained. When a3″ substrate 2904 is utilized, a 3″ GaN film 2924 may be obtained. Whena 4″ substrate 2904 is utilized, a 4″ GaN film 2924 may be obtained. Thethicknesses of the respective GaN layers 2912 and 2916 grown in the twosteps is in one example in a ratio between about 2:1 and about 1:5, andin another example in a ratio between about 1:1 and about 1:3, and inanother example in a ratio between about 1:1 and about 1:2. The exactconditions of the two steps may strongly depend on the reactorconfiguration and method of temperature measurement, and may be easilyfound by those skilled in the arts. The total thickness of the GaN film2924 in one example ranges from approximately 10 to approximately 250microns, in another example from approximately 10 to approximately 200microns, and in another example from approximately 20 to approximately100 microns, and in another example from approximately 20 toapproximately 50 microns.

The HVPE GaN layers 2912 and 2916 may be grown without intentionallyintroduced impurities. However, because of the crystal defects andresidual impurities such as oxygen and silicon from the reactor, anunintentionally doped GaN layer may still have n-type conductivity. TheGaN may also be grown with the presence of intentionally introducedimpurities such as silane or oxygen for n-type doping or magnesium forp-type doping. When transition metal impurities are introduced, the GaNfilm 2924 can be made semi-insulating. Transitional metal impurities,such as iron, may be introduced using, for example, volatilemetal-organic compounds such as ferrocene. It will be understood thatthe growth conditions may be slightly different when the dopingimpurities are introduced. In one example, the dopant concentration(e.g., n-type, p-type, transition metal, etc.) is greater than about1×10¹⁸ cm⁻². In one example of a semi-insulating GaN film 2924 producedaccording to the present disclosure, the GaN film 2924 has a resistivitygreater than about 1×10⁵ ohm-cm.

Because of the thermal mismatch between the substrate 2904 and the GaNfilm 2924, the wafer is bowed after cool-down from the growthtemperature to the ambient temperature. The bow of the wafer complicatesthe device fabrication process and a large bow of the wafer is notdesirable. One aspect of the present invention is that the GaN materialduring growth develops a tensile stress that will compensate the thermalstress and reduce the wafer bow. The tensile stress of the GaN materialduring the growth is associated with the reduction of dislocations inthe GaN material. In another implementation of the present invention, athicker substrate 2904 may also be employed to reduce the GaN film bow.In another implementation, the backside of the substrate 2904 ismechanically lapped to introduce damage on the backside of the substrate2904, which reduces the bow of the GaN film 2924 on the substrate 2904.In one example, the wafer bow is less than about 200 microns. In anotherexample, the wafer bow is less than about 100 microns. In anotherexample, the wafer bow is less than about 50 microns. In anotherexample, the wafer bow is less than about 25 microns. Wafer bow may bedefined as the deviation of the center point of the median surface ofthe wafer from a median-surface reference plane of the wafer.

As an example of wafer bowing, FIG. 31 illustrates a bowed wafer 3104having a bowed median surface 3108 with a center point 3112. A mediansurface reference plane 3116 with a center point 3120 is established bythree equally-spaced points on the median surface at the wafercircumference. In this example, the wafer bow b, projected to the rightof the bowed wafer 3104, is the distance between the center point 3112in the median surface of a free unclamped wafer and the center point3120 in the median surface reference plane 3116. It will be understoodthat the radius of curvature of the bowed wafer 3104 as depicted in FIG.31 is exaggerated for illustrative purposes.

The crystal defect density, specifically, threading dislocation density,decreases with the thickness of the GaN film grown. In implementationsdescribed in the present disclosure, the lattice mismatch between theGaN material and substrate that generates dislocation is firstaccommodated by the AlN layer. The dislocations in the GaN material arefurther annihilated during the two-step GaN growth. The reduction ofdislocation density during HVPE GaN growth according to implementationsdescribed in the present disclosure is much faster than those disclosedin the prior arts. For example, U.S. Pat. Nos. 6,533,874 and 6,156,581disclose a GaN base structure grown by an HVPE process. According to theprior art, the dislocation density of a 10-micron thick GaN film grownby HVPE on sapphire is approximately 10⁹ cm⁻², and the dislocationdensity is reduced to approximately 10⁸ cm⁻² for a 23-micron GaN film,and to approximately 10⁷ cm⁻² for a 300-micron GaN film. Inimplementations of the present invention, improved GaN films have beengrown by HVPE on sapphire, as represented by the following examples: adislocation density on the surface less than 10⁸ cm⁻² for a 10-micronGaN film, less than 5×10⁷ cm² for a 20-micron GaN film, and less than2×10⁷ cm⁻² for a 50-micron GaN film. The surface dislocation density ofGaN film grown according to implementations of the present invention isapproximately several factors lower than GaN films of the prior art atsimilar thickness. According to some examples of the invention, thethreading dislocation density on the surface of the GaN film may be lessthan 1×10⁸ cm⁻², in other examples less than 5×10⁷ cm⁻², in otherexamples less than 1×10⁷ cm⁻², and in other examples less than 5×10⁶cm⁻².

The wafer structure and method for making the structure of the presentinvention differ substantially from the prior art of U.S. Pat. Nos.6,533,874 and 6,156,581. We were not able to grow device-qualityepitaxial single-crystal GaN films using the methods taught by prior artsuch as in these patent references. By contrast, in accordance with thepresent invention, including the use of the epitaxial nitride templatelayer 2908 (FIG. 29) described above, we can reproducibly growdevice-quality epitaxial single-crystal GaN films by HVPE. Additionally,the present invention discloses methods for eliminating GaN filmmicrocracking during HVPE GaN growth. Microcracking of GaN film duringthe HVPE growth and methods for eliminating the growth microcrackinghave not been disclosed in the prior art. Implementations of the presentinvention employ a two-step HVPE GaN growth process to eliminate thegrowth microcracking and to produce smooth surfaces on the GaN films.

Low-defect single-crystal film of Group III nitride alloys,Al_(x)Ga_(y)In_(z)N (x+y+z=1, 0≦x≦1, 0≦y≦1, 0<z<1), may be similarlygrown according to additional embodiments of the present invention. AnAlN nucleation layer is first deposited on a substrate. Single-crystalAl_(x)Ga_(y)In_(z)N film is grown on the AlN-coated substrate by HVPEusing the two-step growth process described above. TheAl_(x)Ga_(y)In_(z)N film is grown under a condition that yields a pittedsurface morphology in the first step and then under a growth conditionthat promotes filling the pits to produce a smooth surface morphology inthe second step. Typically, the first step has a lower growthtemperature, and/or higher growth rate, and/or higher ammonia flow thanthe second growth step. The exact condition for the two-stepAl_(x)Ga_(y)In_(z)N growth depends on the reactor configuration and filmcomposition, and may be easily determined by those skilled in the art.Thus, as previously noted, the term “GaN” encompasses“Al_(x)Ga_(y)In_(z)N.”

The surface morphology of the low-defect GaN film 2924 may be furtherimproved by using a chemical mechanical polish (CMP). The as-grown HVPEGaN film may exhibit some hillock features as shown in FIG. 32. In someapplications, the macroscopic roughness of the GaN film surface 2918(FIG. 29) is less desirable for further device layer growth. The GaNfilm surface 2918 may be improved by chemical mechanical polish. The CMPprocess does not produce surface and subsurface damage on the GaN filmsurface 2918 because of the active chemical etching during the polish.

The present invention can be further understood by followingillustrative, non-limiting examples.

EXAMPLE 9 Low-defect GaN Film Growth

In this example, we illustrate the growth of a high-quality, low-defectGaN film suitable for the further growth of electronic andoptoelectronic devices. A 2″-diameter, 430-micron thick sapphire wasused as the starting substrate. Using the sputtering method disclosed inU.S. Pat. No. 6,784,085, an AlN layer approximately 0.7 μm thick wasgrown on the sapphire substrate for use as a template layer for the HVPEGaN growth. X-ray diffraction was used to verify the AlN film wassingle-crystal. The AlN/sapphire structure was loaded into a verticalHVPE system and the GaN growth was commenced.

The HVPE GaN film was grown by a two-step method. The GaN film was firstgrown under conditions of growth rate of approximately 260 microns perhour, growth temperature of 955° C., HCl flow rate of 92 sccm, and NH₃flow rate of 2500 sccm. After growth of approximately 4 minutes underthese growth conditions, the growth rate was reduced to approximately 65microns per hour by reducing HCl flow to 23 sccm, and growth temperaturewas raised by 20° C. After growth of approximately 7 minutes under theseconditions, the NH₃ flow was further reduced to 750 sccm forapproximately 32 minutes. The total grown GaN film thickness wasapproximately 60 microns. The bow of the wafer was approximately 190microns. The GaN film was specular visually, and under opticalmicroscope observation, hillock features were present on the surface asshown in FIG. 32.

An atomic force microscope (AFM) was used to image the wafer surface andto measure the threading dislocation density. A threading dislocationterminates on the surface as a pit that can be observed with AFM. FIG.33 is a 10-micron by 10-micron AFM scan of the wafer surface. The pitdensity, i.e., the threading dislocation density on the surface, wasapproximately 1.9×10⁷ cm⁻².

EXAMPLE 10 Low-defect GaN Film Growth

In this example, we illustrate the growth of another high-quality,low-defect GaN film suitable for the further growth of electronic andoptoelectronic devices. A 2″-diameter 430-micron thick sapphire was usedas the starting substrate. Using the sputtering method disclosed in U.S.Pat. No. 6,784,085, an AlN layer approximately 0.7 μm thick was grown onthe sapphire substrate for use as a template layer for the HVPE GaNgrowth. The AlN/sapphire structure was loaded into a vertical HVPEsystem and the GaN growth was commenced.

The HVPE GaN film was grown by a two-step method. The GaN film was firstgrown under conditions of growth rate of approximately 260 microns perhour, growth temperature of 955° C., HCl flow rate of 92 sccm, and NH₃flow rate of 2500 sccm. After growth of approximately 3 minutes underthese growth conditions, the growth rate was reduced to approximately 30microns per hour by reducing the HCl flow rate to 10 sccm. At the sametime, the growth temperature was raised by 20° C. and the NH₃ flow ratewas reduced to 400 sccm for an additional 25 minutes. The total grownGaN film thickness was approximately 25 microns. The bow of the waferwas approximately 95 microns. The GaN film was specular visually, andunder optical microscope observation hillock features were present onthe surface.

EXAMPLE 11 Low-defect GaN Film Growth with Lapping Treatment

The GaN film on sapphire obtained from Example 10 is mounted on astainless steel plate using wax with the GaN film facing the plate. Thebackside of the sapphire substrate is lapped on a metal lapping platewith 30-micron diamond slurry. After removing approximately 10 micronsfrom the backside of the sapphire substrate, the wafer bow is reducedfrom approximately 95 microns to approximately 40 microns.

EXAMPLE 12 Low-defect GaN Film Growth with Polishing Treatment

The GaN film on sapphire obtained from Example 11 is mounted on astainless steel plate using wax with the GaN film facing up. The surfaceof the GaN film is then chemical mechanically polished to removeapproximately one micron of surface material. The root-mean square (RMS)surface roughness of the GaN film is reduced from approximately 5 nm forthe as-grown film to approximately 0.5 nm or less for the CMP polishedsurface.

Inclusion-Free Uniform Semi-Insulating Group III Nitride Substrates andMethods for Making Same

As noted above, semi-insulating GaN substrates can be fabricated whendeep-level impurities are introduced during the growth process. An ironimpurity, for example, may be introduced into the reactor vessel byusing a suitable iron source such as, for example, ferrocene(bis(cyclopentadienyl)iron).

FIG. 16 is an optical picture of an as-grown iron-doped GaN substrategrown according to methods described above. The iron-doped GaN issemi-insulating as measured with contactless resistivity mapping(COREMA) and Hall measurement techniques. A 10 mm×10 mm polished GaNwafer was obtained from the wafer and the resistivity of the piece wasmeasured. FIG. 19 shows the Hall resistivity and carrier concentrationas a function of temperature. The room temperature resistivity of thesample was greater than about 2×10⁸ ohm-cm. FIG. 20 is a COREMAresistivity map of a 10 mm×10 mm GaN substrate obtained from theiron-doped GaN wafer. The mean room temperature resistivity was about1×10⁹ ohm-cm and the minimum resistivity measured on the sample wasabout 5×10⁸ ohm-cm.

The GaN obtained according to methods described above still has a fewpits as shown in FIG. 16. These pits may be removed by mechanicallygrinding or lapping away the surrounding materials and polishing thesurface to a mirror finish. FIG. 34 shows a photograph of a polished GaNwafer, showing some inclusions of brown or darker materials in thewafer. The brown inclusions correspond to the areas where pits werepresent prior to the polishing. The inclusions are still single-crystalgallium nitride, but with different impurity levels, in particular, withhigher oxygen impurity in the inclusion than the surrounding area.

The resistivity of the iron-doped GaN wafers with a few brown inclusionswere measured with Hall and COREMA methods, and were found besemi-insulating (above 1×10⁸ ohm-cm). The resistivity of the wafers wasalso measured with the Lehighton method using a LEI 1500 sheetresistance mapping system manufactured by Lehighton Electronics, Inc.The Lehighton method is a known method for measuring the sheetresistance of semi-insulating wafers, such as semi-insulating GaAs. Whenthe sheet resistance of a wafer above 1×10⁵ ohm is measured with theLehighton method, it is considered off-scale for the Lehightonmeasurement, and the wafer is considered as semi-insulating. We measureda set of polished wafers that contained a few brown inclusions with theLehighton method and found that typical sheet resistance was between 50and 5000 ohm/sq, and resistivity was between 2.5 and 250 ohm-cm, muchlower than the value obtained with Hall and COREMA measurements.

The discrepancy between the Lehighton measurement and the COREMAmeasurement may be explained by the physical nature of the measurementtechniques. The Lehighton measurement is based on eddy current andmeasures the sampling area in parallel, whereas the COREMA measurementis based on capacitance and measures the sampling area in series. If theresistivity within the sampling area is not uniform, the resistivitydata obtained by the Lehighton technique will be dominated by thelow-resistivity area, whereas the resistivity data obtained by COREMAmeasurement will be dominated by the high-resistivity area. The brownspots (inclusions) have higher donor impurity concentration and lowerresistivity. The inclusion of low-resistivity material reducesperformance and yield of the electronic devices grown on the substrate.

Methods for Producing Inclusion-free Uniform Semi-insulating GaN

The present invention discloses methods for producing uniformsemi-insulating gallium nitride, without inclusion of conductive spots.In one aspect of the present invention, the morphology of the growthsurface is essentially pit-free during the HVPE growth ofsemi-insulating GaN on c-plane substrate. The presence of pits duringthe SI GaN growth is the source of the inclusion of conductive spots.The elimination of the pits during the growth eliminates the conductivespots in the SI GaN substrates.

In one implementation, the uniform semi-insulating GaN without inclusionof conductive spots is made from HVPE GaN growth on a sapphiresubstrate. The growth process is similar to SI GaN growth methodsdescribed above, but with improvements that ensure complete eliminationof pits during the later stage of the growth. The present method will bedescribed by with reference to FIGS. 35 and 36.

Referring to FIG. 35, a suitable substrate 3504 is provided. In someimplementations, the substrate 3504 may have a characteristic dimension(e.g., diameter) of about 2 inches or greater. As further examples, thediameter of the substrate 3504 may be about 3″ or greater, about 4″ orgreater, or any other suitable size such as about 12″ or greater. Thesubstrate 3504 may be sapphire (Al₂O₃), although other suitablesingle-crystal substrates 3504 may be utilized.

The surface of the substrate 3504 may be exactly c-plane or vicinalsurfaces of the c-plane. Vicinal surfaces may promote step-flow duringthe HVPE GaN growth and may yield smoother surface morphology. Theoffcut angle of the vicinal surface with respect to the c-plane is inone example between about 0.1° and about 10°, and in another examplebetween about 0.5° and about 5°. The direction of offcut may be alongthe <1-100> direction or along the <11-20> direction, or along adirection between <1-100> and <11-20>.

An epitaxial nitride (e.g., AlN) layer 3508 is then deposited on thesubstrate 3504. The purpose of this epitaxial nitride layer 3508 is toprovide a template for epitaxial growth of GaN. The deposition ofepitaxial nitride layer 3508 may be done in the same reactor as for thesubsequent GaN growth, or in a different deposition chamber. In oneimplementation, the epitaxial nitride layer 3508 is deposited byreactive sputtering on a heated substrate in a sputter depositionchamber. The nitride-coated substrate 3504/3508 is subsequently removedfrom the sputter chamber and loaded into the HVPE reactor for GaNgrowth. Other nitride layers such as, for example, AlN grown by MOCVD orMBE, GaN grown by MOCVD or MBE, AlGaN grown by MOCVD or MBE, or the likemay also be used. In some implementations, a reactivesputtering-deposited AlN layer has an advantage of lower cost thanMOCVD- or MBE-deposited nitride layers. AlN layers may also be grown inthe HVPE reactor by incorporating an Al source so that hydrochloric acidreacts with Al to form aluminum chloride, which reacts with ammonia inthe deposition zone to form AlN on the substrate surface. In oneexample, the thickness of the epitaxial nitride layer 3508 is in therange (ranges) from about 0.01 to about 2 microns. In another example,the thickness of the epitaxial nitride layer 3508 ranges from about 0.05to about 10 microns. In another example, the thickness of the epitaxialnitride layer 3508 ranges from about 0.05 to about 2 microns. In anotherexample, the thickness of the epitaxial nitride layer 3508 ranges fromabout 0.2 to about 2 microns.

In some implementations, depending on such factors as process conditionsand the specific reactor being employed, the deposition of the epitaxialnitride layer 3508 may be desirable for growing single-crystal GaN filmson sapphire substrates using the HVPE process. In other HVPE reactorsystems, however, the single-crystalline GaN layer 3512 may be growndirectly on the substrate 3504 by HVPE without using a template layersuch as the epitaxial nitride layer 3508 prior to HVPE growth. In suchcases, the growth sequence according to the teaching of the presentinvention can skip the nitride-coating step and proceed directly to thenext growth step.

The second step of the growth process is to grow an epitaxial GaN layer3512 via 3D growth mode by hydride vapor phase epitaxy with a growthcondition that yields a pitted surface morphology. The uncoatedsubstrate 3504 or nitride-coated substrate 3504/3508 is loaded into anHVPE reactor, and the reactor may be purged with high-purity nitrogen toremove impurities. An epitaxial layer 3512 of gallium nitride is thengrown. This GaN layer 3512 is grown in a three-dimensional (3D) growthmode, where the surface of the film is very rough and covered with pits.The purpose of this single-crystal but pitted GaN layer 3512 is toprevent future microcracking during growth. Without this pitted, roughGaN layer 3512, a smooth GaN layer grown on an AlN-coated sapphiresubstrate typically microcracks when the thickness of the smooth layeris greater than about 20 microns. The growth condition for the pittedlayer 3512 is typically a higher growth rate, and/or higher ammonia flow(or V:III ratio), and/or lower growth temperature than the optimalthin-film growth condition that would produce a smooth surfacemorphology.

In one example, the thickness of the 3D growth layer 3512 ranges fromabout 5 to about 100 microns. In another example, the thickness of the3D growth layer 3512 ranges from about 10 to about 50 microns. Inanother example, the thickness of the 3D growth layer 3512 ranges fromabout 20 to about 30 microns. In another example, the thickness of the3D growth layer 3512 is about 20 microns. The surface coverage of pits,which is defined as the percentage of the area covered with pits overthe total c-plane growth area, is in one example greater than about 50%,in another example greater than about 75%, and in another examplegreater than about 90% at the end of this step. The distribution ofpits, i.e., the size of pits and the depth of pits, is preferably anormal distribution. The aspect ratio of the pits, defined as the depthof the pit divided by the diameter of the pit, is in someimplementations preferred to be a constant. Additional gaseoushydrochloric acid that does not go through the gallium metal (i.e., doesnot produce GaCl) may be added to the reactor and the flow rate of thegas controlled to achieve a constant aspect ratio of the pits. Adeep-level compensating dopant, such as iron, may be optionallyintroduced in this step.

In one implementation, the growth temperature in the 3D growth moderanges from about 900° C. to about 1000° C., the V:III ratio in the 3Dgrowth mode ranges from about 10 to about 100, and the growth rate inthe 3D growth mode ranges from about 50 μm/hr to about 500 μm/hr.

The third step of the growth process changes the HVPE growth conditionsto transition the surface morphology from a heavily pitted surfacemorphology to a gradually less pitted surface morphology. The transitionlayer 3516 is grown under conditions such as lower growth rate, and/orlower ammonia flow, and/or higher growth temperature than the growthcondition of the 3D nucleation layer 3512. In one example, the thicknessof this morphology transition layer 3516 ranges from about 5 to about500 microns. In another example, the thickness of the transition layer3516 ranges from about 10 to about 200 microns. The purposes of thetransition layer 3516 are to prevent the GaN film from turningpolycrystalline, and to prepare the film with a stress state thatfacilitates the formation of lateral cracks during cool-down.

At the end of growth of the transitional layer 3516, the growth surfaceis substantially pit-free. Because nucleation growth conditions are usedin the second step that yield pits with preferably uniformcharacteristics, such as same aspect ratio and crystal orientations ofthe faces of the pits, most pits can be closed during the growth of thetransitional layer 3516. The surface coverage of pits at the end ofgrowth of the transitional layer 3516 is in one example less than about10%, in another example less than about 5%, and in another example lessthan about 1%. Deep-level compensating doping, such as with iron, may beintroduced in this step. Iron doping may be achieved, for example, byintroduction of a volatile metal organic compound such as ferrocene intothe reactor.

In one implementation, the growth temperature in the transitional layergrowth mode ranges from about 920° C. to about 1100° C., the V:III ratioin the transitional layer growth mode ranges from about 8 to about 80,and the growth rate in the transitional layer growth mode ranges fromabout 50 μm/hr to about 500 μm/hr.

The fourth growth step is the bulk growth step where the bulk of the GaNfilm is grown. As illustrated in FIG. 35, a bulk layer or crystal 3520is grown on the transition layer 3516. The growth condition is chosen tofurther eliminate any remaining pits so that the morphology of the GaNfilm is essentially pit-free and the GaN growth mode in this step issubstantially a 2D growth mode. The bulk growth conditions may be lowerammonia flow and/or higher growth temperature than employed during thetransitional layer step. At the end of the bulk growth, the GaN surfaceis essentially pit-free. The density of surface pits at the end of bulkgrowth is in one example less than about 5 cm⁻², in another example lessthan about 1 cm⁻², and in another example about 0 cm⁻². The thickness ofthe GaN grown in the fourth bulk growth step in one example ranges fromabout 500 to about 2000 microns (about 0.5 to about 2 mm), and inanother example ranges from about 1000 to about 1500 microns (about 1 toabout 1.5 mm). Deep-level compensating doping, such as iron, may beintroduced in this step. Iron doping may be achieved, for example, byintroduction of a volatile metal organic compound such as ferrocene intothe reactor.

In one implementation, the growth temperature during the bulk growthstep ranges from about 950° C. to about 1100° C., the V:III ratio duringthe bulk growth step ranges from about 5 to about 50, and the growthrate during the bulk growth step ranges from about 50 μm/hr to about 500μm/hr.

Referring to FIG. 36, after completing the growth, the thickGaN-on-substrate bi-layer 3600 is gradually cooled down. The coolingrate is in one example less than about 20° C. per minute, and in anotherexample less than about 10° C. per minute. In another example, the rateof cooling is about 6° C. per minute. During this cool down time,lateral cracking occurs in the GaN film with the crack plane essentiallyparallel to the GaN/sapphire interface, leading to the separation of GaNfrom the underlying substrate. A thick GaN article or wafer 3622 havinga characteristic dimension (e.g., diameter) as large as the initialsubstrate 3504 (FIG. 35) may be obtained, along with the substrate 3606covered with a thin layer of GaN. For instance, when a 2″ substrate 3504is utilized, a 2″ GaN article 3622 may be obtained. When a 3″ substrate3504 is utilized, a 3″ GaN article 3622 may be obtained. In someexamples, the GaN article 3622 may be broken into several large piecesduring cool down, in which case large pieces may be sized into desiredwafer shapes such as a circle (optionally with flat) or rectangle(optionally with flat).

The thick, freestanding GaN wafers 3622 may be further processed intouniform semi-insulating GaN wafers by using lapping, polishing andchemical mechanical polishing. Because the GaN layer 3520 was firstgrown with a pitted surface morphology which has more oxygenincorporation and thus lower resistivity on the backside, the moreconductive layer near the backside of the article 3622 may be completelyremoved to ensure the semi-insulating nature of the wafer. In oneexample, the freestanding GaN article 3622 is first sized into a desiredwafer shape (herein defined as a wafer blank), optionally with major andminor flats to indicate the crystal orientation of the substrate. In oneexample, the sized GaN wafer blank is about 10 mm×10 mm square orgreater—i.e., the sized GaN wafer blank includes a side having a lengthof about 10 mm or greater. In another example, the sized GaN wafer blankis about 18 mm×18 mm square. In another example, the sized GaN waferblank is about 1 inch round or greater—i.e., the sized GaN wafer blankis circular and has a diameter of about 1 inch or greater. In anotherexample, the sized GaN wafer blank is about 2 inches round or greater.The front of the wafer blank is the Ga-face and the back of the waferblank is the nitrogen-face of the single-crystalline GaN. Material maybe removed from the back side of the wafer blank by mechanical meanssuch as grinding and/or lapping. The thickness removed from the backside of the wafer blank may be at least as much as or greater than thethickness of the grown transitional layer. The front side (Ga face) maybe further polished with diamond slurry. The Ga-surface may be finishedwith a chemical-mechanical polish step that removes the surface andsubsurface damage and produces an epi-ready surface. The back side ofthe wafer blank may be processed by mechanical means such as grinding orlapping to planarize (mechanically flatten) the surface and to achievethe desired wafer thickness. Since the crystal defect density is reducedduring the growth of single-crystalline GaN, it may be preferable totake away material from the back side to achieve the desired waferthickness. Optionally, the back side may be polished to produce anoptical finish.

FIG. 37 shows a photograph of an inclusion-free uniform semi-insulatingGaN wafer produced according to one implementation of the invention. Nobrown inclusion is present on the wafer. Lehighton resistivity mappingmeasurement of the wafers produced with the methods described in thisdisclosure shows that the sheet resistance of the wafer is off-scale ofthe instrument, and thus the wafers are semi-insulating in nature.

In another implementation of the present invention, a native GaN seed isutilized for making inclusion-free semi-insulating GaN wafers. Thec-plane GaN seed wafer is chemical-mechanically polished to remove anyresidual surface and subsurface damage from the mechanical polishprocess. After a thorough cleaning of the seed wafer, it is loaded intoa HVPE reactor. Gallium nitride film is grown on the GaN seed by theHVPE process. Ferrocene as an iron-doping source is introduced foryielding semi-insulating GaN. The growth condition is chosen to ensurethat the surface of the growing GaN crystal (growth surface) remainspit-free during the entire growth process. Typical growth conditions forthis implementation are: growth rate about 100-400 microns per hour,growth temperature about 950-1050° C., NH₃/GaCl (V/III) ratio about5-50, and reactor pressure about 100-760 Torr. It will be understood,however, that the various growth conditions are not limited to theforegoing ranges. The exact conditions for the growth depend on thereactor configuration, and may be determined by those who are familiarwith the art. An aspect of the invention is to maintain the growthsurface and morphology pit-free during the entire growth process. Undera condition of lower temperature and/or higher V/III ratio than theoptimal growth condition window, pits can be formed on the growthsurface, which is undesirable. The iron-doped GaN crystal is grown onthe GaN seed crystal by hydride vapor phase epitaxy under optimal growthconditions that maintain the pit-free growth surface morphology. In oneexample, the length of the grown GaN crystal is about 1 mm or greater.In another example, the length of the GaN crystal is about 5 mm orgreater. In another example, the length of the GaN crystal is about 1 cmor greater. The grown GaN crystal may be sliced into multiple wafers,which may be further processed into epi-ready GaN wafers by mechanicaland chemical-mechanical polishing. The density of the brown spotinclusion is minimal. In one example, the density of the grown spotinclusion is equal to or less than about 1 cm⁻². In another example, thedensity of the brown spot inclusion is about 0 cm⁻².

In conjunction with the above-described implementations, the iron dopinglevel in the iron-doped semi-insulating GaN crystal may be controlled bythe partial pressure of the volatile iron compound in the gas flow. Whenferrocene is used as the iron source, the amount of ferrocenetransported from the ferrocene bubbler to the reactor is determined bythe bubbler temperature and carrier gas flow. In general, the higherconcentration of the volatile iron compound in the gas phase correspondsto a higher concentration of iron in the grown GaN crystal. The exactrelationship between the iron concentration in the GaN crystal and theflow of iron compound depends on the reactor configuration, and can bedetermined by those skilled in the art. The iron concentration should behigher than the total concentration of n-type impurities and defects. Inone example, the iron concentration ranges from about 10¹⁶ to about 10¹⁹cm⁻³. The iron-doped GaN is semi-insulating. In one example, theresistivity is greater than about 1×10⁷ ohm-cm.

Other deep-level acceptors may alternatively be introduced into thegallium nitride crystal by using proper precursors. For instance,transition metals (e.g., Fe, Ni, Co, Mn, Cr, etc.) may be introducedinto the HVPE reactor by the respective metal-organics, or by reactingthe respective metal element with gaseous hydrochloric acid.

The n-type impurities such as silicon and oxygen in the GaN crystalshould be reduced to a minimum. Some precautionary steps may be used toreduce oxygen impurities in the GaN crystal grown by HVPE methods. Thereactor should be leak-tight. A sufficient purge time should be allowedafter loading the substrate into the reactor and prior to crystalgrowth. A load-lock system may be employed for the substrate loading toprevent the exposure of the reactor interior to the ambient air duringloading of the substrate. Purification of gas sources to remove oxygenimpurities may be implemented. In one example, the concentration ofn-type impurities is less than about 10¹⁷ cm⁻³, in another example lessthan about 5×10¹⁶ cm⁻³, and in another example less than about 10¹⁶cm⁻³.

Certain implementations of the present invention may be furtherunderstood by following illustrative, non-limiting example.

EXAMPLE 13 Semi-insulating GaN Growth

In this example, we illustrate a method for making an inclusion-freeuniform semi-insulating GaN substrate. Sapphire(0001) was used as thestarting substrate. An AlN layer approximately 0.25 μm thick was grownon the sapphire substrate by carrying out reactive sputtering on theheated substrate. X-ray diffraction was used to verify the AlN film wassingle-crystal with full width at half maximum (FWHM) of 55 arcsec. TheAlN/sapphire structure was loaded into the previously described verticalHVPE system 100 (FIG. 1) and the GaN growth was commenced.

The HVPE GaN film was grown by a multiple-step method. The GaN film wasfirst grown under conditions of growth rate of approximately 260 micronsper hour, growth temperature of 968° C., HCl flow rate of 92 sccm, andNH₃ flow rate of 2000 sccm. This layer was grown on the AlN-coatedsurface under a 3D growth mode. The growth time for the nucleation layerwas approximately 4 minutes and the thickness of this layer wasapproximately 18 microns. In some runs, the growth was stopped at thispoint, and the wafer was taken out of the reactor for examination. Thewafer surface was visually not specular, and appeared rough. Undermicroscope examination, the surface was covered with a high density ofpits.

After growth of the nucleation layer, the growth rate was reduced toapproximately 65 microns per hour by reducing HCl flow to 23 sccm whilekeeping the same NH₃ flow and growth temperature. After growth forapproximately 7 minutes under these conditions, the HCl flow wasincreased to 46 sccm, the NH₃ flow was reduced to 1500 sccm and growthtemperature was raised by 15° C. to 983° C. for approximately 1 hour.These two growth conditions were considered as the transitional layergrowth stage where surface morphology was improved with less pits.Ferrocene was introduced in the growth system with a nitrogen carriergas flow of 200 sccm. In some runs, the growth was stopped at this pointand the wafer was taken out of the reactor for examination. The wafersurface was visually specular and smooth. A few pits still remained, butthe percentage of the surface covered by pits was less than 1%. Thegrowth mode was transitioned from the 3D mode of the nucleation to a 2Dgrowth mode during this step.

After the growth of the transitional layer, the NH₃ flow was furtherreduced to 900 sccm for an additional growth time of 7 hours. Ferrocenewas introduced in the growth system with a nitrogen carrier gas flow of200 sccm. This was the bulk growth step.

After completion of the growth sequence, the resulting GaN/sapphirebi-layer was cooled to room temperature at a cooling rate ofapproximately 6° C. per minute. During the cooling process, the sapphiresubstrate completely delaminated from the GaN/sapphire bi-layer, therebyforming a freestanding GaN substrate. FIG. 38 is an optical picture ofthe GaN substrate obtained in this example. The GaN substrate was 2inches in diameter and the thickness of the substrate was approximately1.2 mm. The wafer was broken into two large pieces during the separationprocess. The surface was almost pit-free, which is a significantimprovement when compared to the wafer in FIG. 16 obtained using otherGaN growth methods described above.

The as-grown freestanding GaN was cut into 18×18 mm and 10×10 mm wafers.The front surface (Ga-face) was lightly ground to even the surface andapproximately 700 microns of the back side was ground away to eliminateany potential conductive layer. The front side was further polished witha diamond slurry that had diamond particles of approximately 2-4 micronsin diameter. The GaN was finished with a chemical mechanical polishingprocess to remove any surface and subsurface damage. The finished waferwas approximately 500 microns thick. FIG. 37 is a photograph of a 18×18mm inclusion-free substrate obtained in this example.

The crystal defect density was analyzed by etching with phosphoric acidheated to approximately 240° C. to create etch pits and the density ofthe etch pits was measured with an atomic force microscope. The etch pitdensity, which corresponds to the threading dislocation density, wasapproximately 5×10⁶ cm⁻² for the wafers obtained in this example.

The electric property of the wafers was measured with contactlessresistivity mapping (COREMA) and Lehighton techniques. The roomtemperature resistivity of the sample was greater than 2×10⁸ ohm-cmbased on COREMA measurement. The sheet resistance of the wafer wasgreater than 1×10⁵ ohm/square based on Lehighton measurements. Bothtechniques showed uniform resistive electric properties within thespatial resolution of the instruments. The wafers obtained in thisexample were uniformly semi-insulating, and were absent of brown spotinclusions. On the other hand, some wafers obtained using other methodsdescribed above had inclusion of brown spots and a sheet resistance ofless than 1000 ohm/square measured by the Lehighton method, althoughCOREMA measurements showed resistivity greater than 2×10⁸ ohm-cm.

Single Crystal Group III Nitride Articles and Methods for Producing Sameby HVPE Method Incorporating a Polycrystalline Layer for YieldEnhancement

Methods for Producing GaN Articles with the Use of a PolycrystallineLayer

In some cases, methods of GaN substrate production described above formaking a freestanding substrate may suffer a drawback; namely, GaN mayalso break during the separation, reducing the yield of the process.According to the present disclosure, such methods may be furtherimproved by introducing a mechanically stronger polycrystalline layer tocap the single-crystalline GaN layer, thus improving the yield of theprocess.

FIG. 39 is a schematic, sequential illustration of an example of aprocess 3900 of the present invention. First, a sapphire substrate 3904is provided. An epitaxial nitride (e.g., AlN) layer 3908 is thendeposited on a surface 3906 of the sapphire substrate 3904. Thedeposition of epitaxial nitride layer 3908 may be done in the samereactor as for the subsequent GaN growth, or in a different depositionchamber. GaN material is subsequently deposited on the nitride-coatedsubstrate 3904/3908 by hydride vapor phase epitaxy in multiple stepswith different growth conditions for each step. A first GaN layer 3912is grown under a condition that results in a pitted surface morphology,and such conditions are characterized by relatively higher growth rate,and/or high ammonia flow, and/or lower growth temperature than theoptimal thin-film growth conditions that would yield a smooth surfacemorphology. The first GaN layer 3912 thus has a pitted surface 3914. Asecond GaN layer 3916 is grown from the first GaN layer 3912. The secondGaN layer 3916 functions as a transitional layer that is grown under acondition that gradually fills the pits and yields a much less pittedGaN surface 3918, and such growth conditions are characterized byrelatively lower growth rate, and/or lower ammonia flow, and/or highergrowth temperature than the first pitted growth step. A third GaN layer3920 is then grown on the second GaN layer 3916. The third GaN layer3920 is the bulk layer where the majority of single-crystal GaN isgrown. A fourth GaN layer 3924 is then grown on the third GaN layer3920. The fourth GaN layer 3924 is a polycrystalline GaN layer that isprovided to increase the overall mechanical strength of the entire GaNlayers.

During cooling down from the growth temperature to ambient roomtemperature, the grown GaN film separates from the sapphire substrate3904 via lateral cracking, producing a free-standing crack-free GaNarticle 3932 that includes a single-crystal layer 3936 and thepolycrystalline layer 3924. The polycrystalline GaN material 3924 ismechanically stronger than the single-crystal layer 3936 and reduces thebreakage of the GaN article 3932. The freestanding crack-free GaNarticle 3932 is processed by removing the polycrystalline GaN layer 3924to yield a single crystal GaN wafer 3940.

Continuing with the example illustrated in FIG. 39, in someimplementations, the substrate 3904 may have a characteristic dimension(e.g., diameter) of about 2 inches or greater. As further examples, thediameter of the substrate 3904 may be about 3″ or greater, about 4″ orgreater, or any other suitable size such as about 12″ or greater. Thesubstrate 3904 may be sapphire (Al₂O₃), although other suitablesingle-crystal substrates 3904 may be utilized.

The surface 3906 of the substrate 3904 may be exactly c-plane or vicinalsurfaces of the c-plane. Vicinal surfaces may promote step-flow duringthe HVPE GaN growth and may yield smoother surface morphology. Theoffcut angle of the vicinal surface with respect to the c-plane is inone example between about 0.1° and about 10°, and in another examplebetween about 0.5° and about 5°. The direction of offcut may be alongthe <1-100> direction or along the <11-20> direction, or along adirection between <1-100> and <11-20>.

In one implementation, the epitaxial nitride layer 3908 is deposited byreactive sputtering on a heated substrate in a sputter depositionchamber. The nitride-coated substrate 3904/3908 is subsequently removedfrom the sputter chamber and loaded into the HVPE reactor for GaNgrowth. Other nitride nucleation layers such as, for example, AlN grownby MOCVD or MBE, GaN grown by MOCVD or MBE, AlGaN grown by MOCVD or MBE,or the like may also be used. In some implementations, a reactivesputtering-deposited AlN layer has an advantage of lower cost thanMOCVD- or MBE-deposited nitride layers. AlN layers may also be grown inthe HVPE reactor by incorporating an Al source so that hydrochloric acidreacts with Al to form aluminum chloride, which reacts with ammonia inthe deposition zone to form AlN on the substrate surface. In oneexample, the thickness of the epitaxial nitride layer 3908 is in therange (ranges) from about 0.05 to about 10 microns. In another example,the thickness of the epitaxial nitride layer 3908 ranges from about 0.05to about 2 microns. In another example, the thickness of the epitaxialnitride layer 3908 ranges from about 0.5 to about 2 microns.

In some implementations, depending on such factors as process conditionsand the specific reactor being employed, the deposition of the epitaxialnitride layer 3908 may be desirable for growing single-crystal GaN filmson sapphire substrates using the HVPE process. In other HVPE reactorsystems, however, the single-crystalline GaN layer 3912 may be growndirectly on the substrate 3904 by HVPE without using a template layersuch as the epitaxial nitride layer 3908 prior to HVPE growth. In suchcases, the growth sequence according to the teaching of the presentinvention can skip the nitride-coating step and proceed directly to thenext growth step.

The second step of the growth process is to grow an epitaxial GaN layer3912 by hydride vapor phase epitaxy in a 3D growth mode. The uncoatedsubtrate 3904 or nitride-coated substrate 3904/3908 is loaded into anHVPE reactor, and the reactor may be purged with high-purity nitrogen toremove impurities. An epitaxial layer 3912 of gallium nitride is thengrown. This GaN layer 3912 is grown in a three-dimensional (3D) growthmode, where the surface 3914 of the film is very rough and covered withpits. If the growth is stopped at the end of the first step and wafertaken out of the reactor, the GaN surface 3914 is not specular, as shownin a microphotograph in FIG. 40. The GaN film at this stage is stillsingle-crystalline film as demonstrated by x-ray rocking curvemeasurements. The pit coverage, defined as the percentage of the surfacecovered with the pits on the surface, is in one example greater thanabout 50%, and in another example greater than about 75% at the end ofthis step. In other examples, the pitting percentage is greater thanabout 90%. The growth condition for this layer 3912 is typically highergrowth rate, and/or higher ammonia flow (V:III ratio), and/or lowergrowth temperature than the “optimal” thin-film growth condition. Anexample of an “optimal growth condition” is an HVPE GaN growth conditionthat would yield a pit-free GaN thin film (<3 μm thick) on AlN-coatedsapphire substrate as shown in Example 1.

There are two purposes for this 3D growth layer 3912: first is toprevent microcracking of GaN during the growth, and second is to presentthe GaN film with a certain stress condition that will facilitate thelateral cracks during cool down. In one example, the thickness of the 3Dgrowth layer 3912 may range from about 5 to about 100 microns. Inanother example, the thickness of the growth layer 3912 ranges fromabout 10 to about 50 microns. In yet another example, the thickness ofthe 3D growth layer 3912 ranges from about 20 microns to about 30microns. In another example, the thickness of the 3D growth layer 3912is about 20 microns.

In one implementation, the growth temperature in the 3D growth moderanges from about 900° C. to about 1000° C., the V:III ratio in the 3Dgrowth mode ranges from about 10 to about 100, and the growth rate inthe 3D growth mode ranges from about 50 μm/hr to about 500 μm/hr.

If the GaN film is grown under the 3D growth mode with higher thickness,the GaN film quality is gradually changed from an epitaxialsingle-crystalline film to a polycrystalline film.

The third step of the growth process is to change the HVPE growthconditions to transition the surface morphology from a heavily pittedsurface morphology to a gradually less pitted surface morphology. Thetransition layer 3916 is grown under conditions such as lower growthrate, and/or lower NH₃ flow, and/or higher growth temperature than thegrowth condition of the nucleation layer 3912. In one example, thethickness of this morphology transition layer 3916 ranges from about 5to about 500 microns. In another example, the thickness of thetransition layer 3916 ranges from about 5 to about 200 microns. Inanother example, the thickness of the transition layer 3916 ranges fromabout 5 to about 100 microns. In another example, the thickness of thetransition layer 3916 ranges from about 5 to about 50 microns. Inanother example, the thickness of the transition layer 3916 is about 8microns. The purposes of the transition layer 3916 are to prevent theGaN film from turning into polycrystalline, and to prepare in the film astress state that facilitates lateral cracks during cool down. At theend of growth of the transitional layer 3916, the growth surface 3918 issubstantially pit-free. The surface coverage of pits at the growthsurface 3918 after growing the transitional layer 3916 is in one exampleless than about 10%, in another example less than about 5%, and inanother example less than about 1%.

In one implementation, the growth temperature in the recovery layergrowth mode ranges from about 920° C. to about 1100° C., the V:III ratioin the recovery layer growth mode ranges from about 8 to about 80, andthe growth rate in the recovery layer growth mode ranges from about 50μm/hr to about 500 μm/hr.

The fourth growth step is the bulk growth step where the bulk of thesingle-crystalline GaN film is grown. The growth condition is chosen sothat the morphology of the GaN film remains slightly pitted or pit-free.The GaN growth mode in this step is substantially a 2D growth mode. Thegrowth conditions may be held constant during this step. Alternatively,one or more growth conditions may be slightly ramped, for example,slightly ramping down ammonia flow or slightly ramping down the growthrate or slightly ramping up the temperature. The purpose of the rampingin the bulk growth step is to further reduce the density of the pits onthe growing GaN surface. During the bulk growth step, the density of thepits on the growing GaN surface is gradually reducing. At the end of thebulk growth, the GaN surface 3922 is slightly pitted or pit-free. In oneexample, the thickness of the GaN bulk layer 3920 grown in the bulkgrowth step ranges from about 500 to about 2000 microns (about 0.5 toabout 2 mm). In another example, the thickness of the GaN bulk layer3920 ranges from about 1000 to about 1500 microns (about 1 to about 1.5mm).

In one implementation, the growth temperature in the bulk growth stageranges from about 950° C. to about 1100° C., the V:III ratio in the bulkgrowth stage ranges from about 5 to about 50, and the growth rate in thebulk growth stage ranges from about 50 μm/hr to about 500 μm/hr.

The fifth step is to grow a polycrystalline GaN film 3924 on top of thesingle-crystalline GaN film 3920 grown in the fourth step by changingthe growth condition. The polycrystalline GaN film 3924 may be grown byreducing the growth temperature, and/or increasing the NH₃ flow, and/orincreasing the growth rate, as compared with the growth conditionsemployed in the preceding bulk growth step. In one example, thethickness of the polycrystalline GaN film 3924 ranges from about 500 toabout 2000 microns (0.5 to 2 mm).

In one implementation, the growth temperature in the polycrystallinegrowth stage ranges from about 850° C. to about 1000° C., the V:IIIratio in the bulk growth stage ranges from about 20 to about 200, andthe growth rate in the bulk growth stage ranges from about 100 μm/hr toabout 500 μm/hr.

The polycrystalline layer 3924 is postulated herein to improve theintegrity of the GaN article 3932 in the following manners. In a singlecrystal, the cleavage planes of the crystal are substantially alignedand when a crack forms, there is little to blunt the crack fromprogressing through the thickness or length of the crystal. When apolycrystalline layer 3924 is deposited on the single crystal, thecrystal cleavage planes in the polycrystal are not aligned with theplanes in the single crystal. If a crack were to propagate through thesingle crystal, the energy required to propagate the crack through thepolycrystal increases, thus making the article stronger. Additionally,when a polycrystalline layer 3924 is deposited on top of the singlecrystal, the thickness of the crystal changes and the stress in theentire GaN layer due to thermal expansion mismatch is reduced. Thisreduces the likelihood of crack formation in the article.

After completing the growth, the thick GaN-on-substrate bi-layer isgradually cooled down. The cooling rate is in one example less thanabout 20° C. per minute, and in another example less than about 10° C.per minute. In another example, the rate of cooling is about 6° C. perminute. During this cool down time, lateral cracking occurs in the GaNfilm with the crack plane essentially parallel to the GaN/substrateinterface, leading to the separation of GaN from the underlyingsubstrate. A thick GaN article 3932 having a characteristic dimension(e.g., diameter) as large as the initial substrate 3904 may be obtained,along with the substrate covered with a thin layer of GaN. As examples,when a 2″ substrate 3904 is utilized, a 2″ GaN article 3932 may beobtained. When a 3″ substrate 3904 is utilized, a 3″ GaN article 3932may be obtained. When a 4″ substrate 3904 is utilized, a 4″ GaN article3932 may be obtained. The substrate 3904 may remain intact, or remainpartially intact with edge fracture, or fracture into several pieces.The remaining GaN on the substrate 3904 is typically less than 500microns thick. The resulting freestanding GaN article 3932 is typically1-4 mm thick.

The freestanding GaN article 3932 may be processed into asingle-crystalline GaN wafer or substrate 3940 by mechanical means suchas grinding or lapping and polishing. In one example, the freestandingGaN article 3932 is first sized into a desired wafer shape (hereindefined as a wafer blank), optionally with major and minor flats toindicate the crystal orientation of the substrate 3940. In one example,the sized GaN wafer blank is about 10 mm×10 mm square or greater—i.e.,the sized GaN wafer blank includes a side having a length of about 10 mmor greater. In another example, the sized GaN wafer blank is about 18mm×18 mm square. In another example, the sized GaN wafer blank is about1 inch round or greater—i.e., the sized GaN wafer blank is circular andhas a diameter of about 1 inch or greater. In another example, the sizedGaN wafer blank is about 2 inches round or greater. The front of thewafer blank is the polycrystalline GaN and the back of the wafer blankis the nitrogen-face of the single-crystalline GaN. The polycrystallinepart of the wafer blank may be removed by mechanical means such asgrinding and/or lapping. The thickness removed from the front side ofthe wafer blank may be at least as much as or greater than the thicknessof the polycrystalline material to expose the Ga-face 3944 of the singlecrystalline GaN substrate 3940. After completely removing thepolycrystalline material from the front side of the wafer blank, thefront side (Ga face) may be further polished with diamond slurry. TheGa-surface 3944 may be finished with a chemical-mechanical polish stepthat removes the surface and subsurface damage and produces an epi-readysurface. The back side of the wafer blank may be processed by mechanicalmeans such as grinding or lapping to planarize (mechanically flatten)the surface and to achieve the desired wafer thickness. Since thecrystal defect density is reduced during the growth ofsingle-crystalline GaN, it may be preferable to take away material fromthe back side to achieve the desired wafer thickness. Optionally, theback side may be polished to produce an optical finish.

In some implementations, during the bulk growth, no impurity isintroduced and all the gas sources are purified, in order to achievehigh-purity GaN crystals. In some examples, the impurity concentrationin the high-purity GaN layer is less than about 10¹⁷ cm⁻³. In otherexamples, the impurity concentration is less than about 10¹⁶ cm⁻³. Inother examples, the impurity concentration is less than about 10¹⁵ cm⁻³.

Alternatively, in other implementations, during the bulk growth, n-typeimpurities, such as silicon (introduced, for example, as diluted silane)or oxygen, are introduced to produce n-type GaN crystals. Theintroduction of impurities may occur at any stage of the GaN growth. Insome examples, the electron concentration in the n-type bulk GaN layeris greater than about 10¹⁷ cm⁻³. In other examples, the electronconcentration is greater than about 10¹⁸ cm⁻³. In other examples, theelectron concentration is greater than about 10¹⁹ cm⁻³. In someexamples, the resistivity of the n-type GaN layer is less than about 0.1ohm-cm. In other examples, the resistivity is less than about 0.01ohm-cm. In other examples, the resistivity is less than about 0.001ohm-cm. Conductive GaN wafers may be used as substrates for thefabrication of optoelectronic devices such as light emitting diodes,laser diodes, and photodetectors. Conductive GaN wafers may also be usedas substrates for electronic devices such as Schottky diodes andheterojunction bipolar transistors.

In other implementations, during the bulk growth, p-type impurities suchas magnesium (Mg) are introduced to produce p-type GaN crystals. Mg maybe introduced as metal-organic compound or as metal vapor. Theintroduction of impurities may occur at any stage of the growth. In someexamples, the hole concentration in the p-type bulk GaN layer is greaterthan about 10¹⁷ cm⁻³. In other examples, the hole concentration isgreater than about 10¹⁸ cm⁻³. In other examples, the hole concentrationis greater than about 10¹⁹ cm⁻³. In some examples, the resistivity ofthe p-type GaN layer is less than about 0.1 ohm-cm. In other examples,the resistivity is less than about 0.01 ohm-cm. In other examples, theresistivity is less than about 0.001 ohm-cm.

In other implementations, the electrical properties of the bulk GaNcrystal can also be made semi-insulating by introducing a deep-levelacceptor. Transition metals, such as iron, are deep level acceptors.Iron can be introduced to the deposition zone either via a metal-organiccompound such as ferrocene or via iron chloride formed by reacting ironwith hydrochloric acid. The concentration of the transition metal in thebulk GaN layer may range from about 10¹⁶ to about 10²⁰ cm⁻³. In otherexamples, the concentration of the transition metal ranges from about10¹⁷ to about 10¹⁹. In other examples, the concentration of thetransition metal is around 10¹⁸ cm⁻³. The resistivity of the SI bulk GaNlayer at room temperature may be greater than about 10⁶ ohm-cm. In otherexamples, the resistivity is greater than about 10⁷ ohm-cm. In otherexamples, the resistivity is greater than about 10⁸ ohm-cm.Semi-insulating GaN wafers can be used as substrates for electronicdevices such as high electron mobility transistors.

Certain implementations of the present invention may be furtherunderstood by the following illustrative, non-limiting examples.

EXAMPLE 14 Si-doped GaN Growth

In this example, we illustrate production of silicon-doped 2″ singlecrystalline GaN wafer. A 2″ c-plane sapphire was used as the startingsubstrate. An epitaxial AlN layer was deposited using high temperaturereactive sputtering method, as disclosed in U.S. Pat. No. 6,704,085. Thethickness of the AlN layer was approximately 0.25 μm. X-ray rockingcurve measurement showed a full width at half maximum of about 55 arcsecfor the AlN(0002) reflection, indicating high crystal quality of the AlNfilm. The AlN-coated sapphire substrate was loaded into the previouslydescribed vertical HVPE system and the GaN growth was commenced.

FIG. 41 is an illustration of the HVPE GaN growth process 4100 for thisexample, including the temperature 4104, NH₃ flows 4108, and HCl flows4112 for the nucleation stage 4122, transition stage 4126, bulk growthstage 4130, and polycrystalline growth stage 4134. The growth processincluded a nucleation step 4122 on the AlN-coated sapphire substratewith a rough surface morphology (3D growth mode), transitioning 4126 thegrowth condition from the nucleation stage (3D growth mode) 4122 to thesinglecrystalline bulk growth stage (2D growth mode) 4130,single-crystalline bulk growth 4130, and polycrystalline cap growth4134.

The nucleation layer was grown at a growth temperature of about 995° C.,HCl flow of about 120 sccm, and NH₃ flow of about 4000 sccm,corresponding to a growth rate of about 330 μm/hr. The growth time wasabout 5 minutes, corresponding to a thickness of about 28 μm. In someexamples, the growth was stopped at this stage and the wafer was takenout for examination. The GaN surface was very rough and not specular.Under optical microscope examination, the surface contains high densityof pits, similar to the image shown in FIG. 40.

After the growth of the 3D nucleation layer, the growth condition waschanged to influence the evolution of the surface morphology from aheavily pitted surface to a much less pitted surface. The growthtemperature was raised by about 15° C. to about 1010° C., NH₃ flow wasreduced from about 4000 sccm to about 2000 sccm. The growth rate wasremained at 330 μm/hr and the growth time was 30 minutes, correspondingto a nominal thickness of about 165 μm for the transitional layer.Silane was introduced into the reactor during the growth. In someexamples, the growth was stopped at this stage, and the wafer was takenout of the reactor for examination. The GaN surface was visuallyspecular. When examined under optical microscope, the GaN surfaceexhibited typical hillock morphology with some pits. FIG. 42 shows anoptical micrograph of the GaN film grown at this stage. The surface areaoccupied by the pits was quite low, typically less than 5%.

Following the transition layer, the single crystalline bulk GaN layerwas grown. The growth temperature and the growth rate remained the sameas those for the transitional layer, 1010° C. and 330 μm/hr. The NH₃flow was further reduced from 2000 sccm to 1600 sccm for this stage ofthe growth. The silane flow was not changed. The growth time for thisstep was 3.5 hours, corresponding to a nominal thickness of about 1.1 mmfor the material grown in this step. The total epitaxialsingle-crystalline GaN film grown was about 1.35 mm thick. In someexamples, the growth was stopped at this stage, and the wafer was takenout of the reactor for examination. The surface morphology was similarto that shown in FIG. 42, except that the hillock feature is larger andsurface is almost pit-free. Under these growth conditions, the GaNcracked laterally during cool down and separated the GaN from thesapphire (the sapphire still has a thin GaN layer). Because the GaN alsohas a few cracks from the fracture during cool-down, frequently onlylarge pieces of freestanding GaN and not the whole 2″ substrate wereobtained.

After the growth of the bulk single crystalline GaN layer, the growthcondition was changed to grow a mechanically stronger polycrystallineGaN cap. The growth temperature was reduced by 20° C. to 990° C. and theNH₃ flow was increased from 1600 sccm to 6000 sccm. The HCl flow waskept at 120 sccm for a nominal growth rate of 330 μm/hr. At this growthcondition, the GaN film gradually changed into a polycrystalline filmduring the growth. In this example, the growth time for thepolycrystalline cap was about 4 hours and the nominal thickness of thepolycrystalline GaN material was about 1.3 mm. The total GaN thicknesswas about 2.6 mm.

After completion of the growth sequence, the GaN/sapphire bi-layer wascooled to room temperature at a cooling rate of approximately 6° C. perminute. During the cooling process, the GaN cracked laterally, forming a2″ freestanding GaN article and the sapphire substrate with a thin GaNlayer still attached. Because of the presence of the mechanicallystronger polycrystalline GaN layer, the GaN article did not crackvertically during the laterally cracking separation. The freestanding 2″GaN article was crack-free. The thickness of the freestanding GaNarticle was about 2.1 mm.

The top surface of the freestanding GaN was polycrystalline material andthe bottom surface was the nitrogen face (N-face) of the singlecrystalline GaN. The bottom surface was not flat because it wasgenerated from lateral cracking.

The 2″ freestanding GaN article was processed into 2″ GaN wafer. First,edge grinding was performed to make into within 0.005″ tolerance of the2″ wafer diameter. Because the initial sapphire had a major flat, thegrown GaN article also had a flat. The ground 2″ GaN article is called awafer blank herein.

The 2″ GaN wafer blank was mounted on a fixture using wax with the frontsurface facing the fixture. The back surface (N-face) was lapped on ametal plate with diamond slurry until a uniform surface finish wasachieved. At this stage, the thickness of the blank was about 1.8 mm.The 2″ GaN wafer blank was removed from the fixture and mounted againusing wax with the back side facing the fixture. The front side waslapped on a metal plate with diamond slurry. About 1.3 mm of the frontmaterial was removed. At this stage, the polycrystalline material wascompletely removed, resulting in a 2″ single crystalline GaN wafer about500 micron thick. Because silicon doping was used during the growth, theGaN wafer was conductive with a resistivity less than 0.05 ohm-cm.

EXAMPLE 15 Semi-insulating GaN Growth

In this example, we illustrate production of semi-insulating 2″ GaNsubstrate. Sapphire(0001) was used as the starting substrate. An AlNlayer approximately 0.25 μm thick was grown on the sapphire substrateusing reactive sputtering on the heated substrate. X-ray diffraction wasused to verify the AlN film was single crystal with a full width at halfmaximum of 55 arcsec. The AlN/sapphire structure was loaded into thepreviously described vertical HVPE system and the GaN growth wascommenced.

The HVPE GaN film was grown by a multiple-step method. FIG. 43 is anillustration of the HVPE GaN growth process 4300 for this example,including the temperature 4304, NH₃ flows 4308, and HCl flows 4312 forthe nucleation stage 4322, transition stage 4326, bulk growth stage4330, and polycrystalline growth stage 4334. The growth process 4300included a nucleation step 4322 on the AlN-coated sapphire substratewith a rough surface morphology (3D growth mode), transitioning 4326 thegrowth condition from the nucleation stage (3D growth mode 4322) to thesingle-crystalline bulk growth stage (2D growth mode) 4330,single-crystalline bulk growth 4330, and polycrystalline cap growth4334.

The GaN film was first grown under conditions of growth rate ofapproximately 260 microns per hour, growth temperature of 968° C., HClflow rate of 92 sccm, and NH₃ flow rate of 2000 sccm. This was the GaNnucleation layer on the AlN-coated surface. The growth time for thenucleation layer was approximately 4 minutes and the thickness of thislayer was approximately 18 microns. In some runs, the growth was stoppedat this point, and the wafer was taken out of the reactor forexamination. The wafer surface was visually not specular, and appearedrough. Under microscope examination, the surface was covered with highdensity of pits. This layer was grown under a 3D growth mode.

After growth of the nucleation layer, the growth rate was reduced toapproximately 65 microns per hour by reducing HCl flow to 23 sccm whilekeeping the same NH₃ and growth temperature. After growth ofapproximately 7 minutes under these conditions, the HCl flow wasincreased to 46 sccm, the NH₃ flow was reduced to 1500 sccm and growthtemperature was raised by 15° C. to 983° C. for approximately 1 hour.These two growth conditions were considered as the transitional layerwhere the surface morphology was improved with less pits. Ferrocene wasintroduced in the growth system using a nitrogen carrier gas flow of 200sccm. In some runs, the growth was stopped at this point and wafer wastaken out of the reactor for examination. The wafer surface was visuallyspecular and smooth. A few pits still remained, but the percentage ofthe surface covered by pits was less than 1%. In this step the growthmode was transitioned from the 3D mode of the nucleation to 2D growthmode.

After the growth of the transitional layer, the NH₃ flow was furtherreduced to 900 sccm for additional growth time of 7 hours. The ferroceneflow was maintained during this layer.

After the growth of the bulk single crystalline GaN layer, the growthcondition was changed to grow a mechanically stronger polycrystallineGaN cap. The growth temperature was reduced by 20° C. to 963° C. and theNH₃ flow was increased from 900 sccm to 5000 sccm. The HCl flowincreased to 120 sccm for a nominal growth rate of 330 μm/hr. At thisgrowth condition, the GaN film gradually changed into polycrystallineduring the growth. In this example, the growth time for thepolycrystalline cap was about 4 hours and the nominal thickness of thepolycrystalline GaN material was about 1.3 mm. The total GaN thicknesswas about 2.6 mm.

After completion of the growth sequence, the GaN/sapphire bi-layer wascooled to room temperature at a cooling rate of approximately 6° C. perminute. During the cooling process, the GaN cracked laterally, forming a2″ freestanding GaN article and the sapphire substrate with a thin GaNlayer still attached. Because of the presence of the mechanicallystronger polycrystalline GaN layer, the GaN article did not crackvertically during the laterally cracking separation. The freestanding 2″GaN article was crack-free. The thickness of the freestanding GaNarticle was about 2.1 mm.

The top surface of the freestanding GaN was polycrystalline material andthe bottom surface was the nitrogen face (N-face) of the singlecrystalline GaN. The bottom surface was not flat because it wasgenerated from lateral cracking.

The 2″ freestanding GaN article was processed into 2″ GaN wafer. First,edge grinding was performed to make into within 0.005″ tolerance of the2″ wafer diameter. Because the initial sapphire had a major flat, thegrown GaN article also had a flat. The ground 2″ GaN article is calledwafer blank herein.

The 2″ GaN wafer blank was mounted on a fixture using wax with the frontsurface facing the fixture. The back surface (N-face) was lapped on ametal plate with diamond slurry until a uniform surface finish wasachieved. At this stage, the thickness of the blank was about 1.8 mm.The 2″ GaN wafer blank was removed from the fixture and mounted againusing wax with the back side facing the fixture. The front side waslapped on a metal plate with diamond slurry. About 1.3 mm of the frontmaterial was removed. At this stage, the polycrystalline material wascompletely removed, resulting in a 2″ single crystalline GaN wafer about500 micron thick. Because iron doping was used during the growth, theGaN wafer was semi-insulating with a resistivity greater than 10⁶ohm-cm.

Many examples of the present invention utilized AlN-coated sapphire asthe substrate for the GaN crystal growth. As previously noted, othersubstrates, coated with another type of Group III nitride such as GaN ormore generally (Al, Ga, In)N, may also be used in the present inventionand thus are included within the scope of the invention. In other HVPEreactor systems that enable direct nucleation of single crystalline GaNfilm on bare sapphire substrate, bare sapphire may be used as substratefor the bulk GaN growth in accordance with the scope of the presentinvention. GaN wafers, which may be produced according toimplementations of the present invention, may also be used as thesubstrate for the bulk GaN crystal growth.

Many examples of the present invention utilized several specific growthsequences. It should be understood that these specific growth processesare meant for illustrative purposes and are not limiting. It should alsobe noted that growth conditions cited in the examples are specific tothe HVPE growth reactor employed in the examples. When employing adifferent reactor design or reactor geometry, it may be desirable toutilize a different condition to achieve similar results. However, thegeneral trends are still similar.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the growth of the bulk GaNcrystal within the scope of the present invention. Thus it is construedthat the present invention covers the variations and modifications ofthis invention provided they come within the scope of the appendedclaims and their equivalents.

What is claimed is:
 1. A method for forming a bulk crystal structure,the method comprising: depositing an epitaxial AlN layer on asingle-crystal substrate to form an AlN-coated substrate; growing a GaNnucleation layer on the AlN-coated substrate by HVPE under nucleationlayer growth conditions that produce a pitted nucleation layer surface;growing a GaN transitional layer on the pitted nucleation layer surfaceby HVPE under transitional layer growth conditions that (i) aredifferent from the nucleation layer growth conditions and (ii) produce atransitional layer surface having a lesser percentage of pits than thepitted nucleation layer surface; and growing a GaN bulk layer on thetransitional layer surface by HVPE.
 2. The method of claim 1, whereinthe nucleation layer growth conditions include a first growthtemperature, a first ammonia partial pressure, a first V:III ratio, anda first growth rate, the transitional layer growth conditions include asecond growth temperature, a second ammonia partial pressure, a secondV:III ratio, and a second growth rate, and the transitional layer growthconditions are selected from the group consisting of the first growthtemperature being increased to the second growth temperature, the firstammonia partial pressure being reduced to the second ammonia partialpressure, the first growth rate being reduced to the second growth rate,and two or more of the foregoing.
 3. The method of claim 2, wherein thefirst growth temperature ranges from about 900° C. to about 1000° C.,the first V:III ratio ranges from about 10 to about 100, the firstgrowth rate ranges from about 50 μm/hr to about 500 μm/hr, the secondgrowth temperature ranges from about 920° C. to about 1100° C., thesecond V:III ratio ranges from about 8 to about 80, and the secondgrowth rate ranges from about 50 μm/hr to about 500 μm/hr.
 4. The methodof claim 1, wherein a pitting percentage of the pitted nucleation layersurface is greater than about 50%.
 5. The method of claim 4, wherein adiameter of pits of the pitted nucleation layer surface ranges fromabout 5 to about 50 microns.
 6. The method of claim 1, wherein a pitdensity within the GaN transitional layer decreases as a function of athickness of the GaN transitional layer.
 7. The method of claim 1,wherein the transitional layer surface has a smooth surface morphology,and a surface morphology of the GaN bulk layer remains smooth duringgrowth thereof.
 8. The method of claim 1, further comprising introducingan n-type or p-type impurity during at least one of the growth steps. 9.The method of claim 1, further comprising introducing a deep-levelacceptor during at least one of the growth steps.
 10. The method ofclaim 1, wherein the epitaxial AlN layer is deposited by a techniqueselected from the group consisting of sputtering, MOVPE, MBE, HVPE, andannealing in ammonia.
 11. The method of claim 10, wherein the epitaxialAlN layer is deposited by sputtering.
 12. The method of claim 1, whereinthe GaN nucleation layer is substantially free of cracks andmicrocracks.